Automated laminography system for inspection of electronics

ABSTRACT

A tomographic inspection system wherein the electron beam of a microfocus X-ray tube is deflected in a circular scan pattern onto the tube anode in synchronization with a rotating detector that converts the X-ray shadowgraph into an optical image and derotates the image so as to be viewed and integrated in a stationary video camera. A computer system controls an automated positioning system that supports the item under inspection and moves successive areas of interest into view. In order to maintain high image quality, a computer system also controls the synchronization of the electron beam deflection and rotating optical system, making adjustments for inaccuracies of the mechanics of the system. The computer system can also operate under program control to automatically analyze data, measure characteristics of the item under inspection and make decisions regarding the acceptability of the item&#39;s quality. The invention produces high resolution images in rapid succession so as to be suitable for use in conjunction with manufacturing production lines and capable of inspecting electronic devices, solder connections, printed wiring boards and other assemblies.

This application is a continuation of application Ser. No. 115,171,filed Oct. 10, 1987 now U.S. Pat. No. 4,926,452.

FIELD OF THE INVENTION

The invention relates generally to the art of tomography, especially toa computerized laminography system for rapid, high resolution inspectionof manufactured electronic items.

BACKGROUND OF THE INVENTION

Rapid and precise quality control inspections of the soldering andassembly of electronic devices have become priority items in theelectronics manufacturing industry. The reduced size of components andsolder connections, the resulting increased density of components oncircuit boards and the advent of surface mount technology (SMT), whichplaces solder connections underneath device packages where they arehidden from view, have made rapid and precise inspections of electronicdevices and the electrical connections between devices very difficult toperform in a manufacturing environment.

Many existing inspection systems for electronic devices and connectionsmake use of penetrating radiation to form images which exhibit featuresrepresentative of the internal structure of the devices and connections.These systems often utilize conventional radiographic techniques whereinthe penetrating radiation comprises X-rays. Medical X-ray pictures ofvarious parts of the human body, e.g., the chest, arms, legs, spine,etc., are perhaps the most familiar examples of conventionalradiographic images. The images or pictures formed represent the X-rayshadow cast by an object being inspected when it is illuminated by abeam of X-rays. The X-ray shadow is detected and recorded by an X-raysensitive material such as film or other suitable means.

The appearance of the X-ray shadow or radiograph is determined not onlyby the internal structural characteristics of the object, but also bythe direction from which the incident X-rays strike the object.Therefore, a complete interpretation and analysis of X-ray shadowimages, whether performed visually by a person or numerically by acomputer, often requires that certain assumptions be made regarding thecharacteristics of the object and its orientation with respect to theX-ray beam. For example, it is often necessary to make specificassumptions regarding the shape, internal structure, etc. of the objectand the direction of the incident X-rays upon the object. Based on theseassumptions, features of the X-ray image may be analyzed to determinethe location, size, shape, etc., of the corresponding structuralcharacteristic of the object, e.g., a defect in a solder connection,which produced the image feature. These assumptions often createambiguities which degrade the reliability of the interpretation of theimages and the decisions based upon the analysis of the X-ray shadowimages. One of the primary ambiguities resulting from the use of suchassumptions in the analysis of conventional radiographs is that smallvariations of a structural characteristic within an object, such as theshape, density and size of a defect within a solder connection, areoften masked by the overshadowing mass of the solder connection itselfas well as by neighboring solder connections, electronic devices,circuit boards and other objects. Since the overshadowing mass andneighboring objects are usually different for each solder joint, it isextremely cumbersome and often nearly impossible to make enoughassumptions to precisely determine shapes, sizes and locations of solderdefects within individual solder joints.

In an attempt to compensate for these shortcomings, some systemsincorporate the capability of viewing the object from a plurality ofangles. The additional views enable these systems to partially resolvethe ambiguities present in the X-ray shadow projection images. However,utilization of multiple viewing angles necessitates a complicatedmechanical handling system, often requiring as many as five independent,non-orthogonal axes of motion. This degree of mechanical complicationleads to increased expense, increased size and weight, longer inspectiontimes, reduced throughput, impaired positioning precision due to themechanical complications, and calibration and computer controlcomplications due to the non-orthogonality of the axes of motion.

Many of the problems associated with the conventional radiographytechniques discussed above may be alleviated by producingcross-sectional images of the object being inspected. Tomographictechniques such as laminography and computed tomography (CT) are oftenused in medical applications to produce cross-sectional or body sectionimages. In medical applications, these techniques have met withwidespread success, largely because relatively low resolution on theorder of one or two millimeters (0.04 to 0.08 inches) is satisfactoryand because speed and throughput requirements are not as severe as thecorresponding industrial requirements. However, no laminographyinspection system has yet met with commercial success in an industrialapplication because of shortcomings in precision and/or speed ofinspection. This is because existing laminography systems have beenincapable of achieving the high positional accuracies and imageresolutions necessary to solve industrial inspection problems whileoperating at the speeds necessary to make them practical in a productionenvironment.

In the case of electronics inspection, and more particularly, forinspection of electrical connections such as solder joints, imageresolution on the order of several micrometers, for example, 20micrometers (0.0008 inches) is necessary. Furthermore, an industrialsolder joint inspection system must generate multiple images per secondin order to be practical for use on an industrial production line.Heretofore, laminography systems have not been able to achieve thesespeed and accuracy requirements necessary for electronics inspection.

Laminography systems for the production of cross-sectional images havetaken several forms. One system is described in U.S. Pat. No. 3,928,769entitled "LAMINOGRAPHIC INSTRUMENT." The radiation source and thedetector described therein are mechanically coupled to achieve therequired geometry and synchronized motion of the source and detector.This type of system has the disadvantage of having to move therelatively high mass of some combination of high mass elements includingthe radiation source, object under inspection and detector. This becomesespecially difficult when X-ray tubes and camera equipment are to beused. The speed of this system is severely restricted due to the factthat it is extremely difficult to move these relatively large massesrapidly and precisely. This system also has limitations on theresolution that can be obtained due to the imprecision and degradationover time of the many complicated moving parts.

In another system described in U.S. Pat. No. 4,211,927 entitled"COMPUTERIZED TOMOGRAPHY SYSTEM," the mechanical motions of theradiation source and detector are electronically driven by separatestepper motors whose timing is controlled by the same computer. Themotion of each component is referenced to a respective predeterminedcentral calibration location. Thus, even though the source and detectorare driven by the same computer, there is no direct link correlating theposition of the source with the position of the detector. Theperformance of this system is also limited by the speed at which themassive radiation source and detector can be oscillated and by theprecision, synchronization and stability of the moving parts.

In U.S. Pat. No. 4,516,252 entitled "DEVICE FOR IMAGING LAYERS OF ABODY," a plurality of radiation sources, each fixed in space at adifferent location, is used in lieu of a single oscillating source. Thelocation of an image detector is moved electronically in synchronizationwith the activation of the plural sources. While this approacheliminates the problems inherent with mechanically moving the radiationsource and detector, it entails the disadvantage in cost of requiringmultiple radiation sources. The resulting image quality is also degradedbecause the desired blurring of out of focus features is not continuous,but rather discretized, due to the finite number of radiation sourcepositions. Thus, unwanted features remain in the image as a plurality ofdistinct artifacts.

U.S Pat. No. 2,667,585 entitled "DEVICE FOR PRODUCING SCREENING IMAGESOF BODY SECTIONS" shows a stationary X-ray tube with the radiationsource motion provided by electrostatic deflection of the electron beamin the X-ray tube, thus causing the electron beam to trace a path overthe surface area of a flat target anode. Opposite the X-ray tube is adetector image tube containing electron optics which deflect theresulting electron image onto a stationary detector. The deflectioncircuit of the X-ray tube and the deflection circuit of the image tubeare driven from the same voltage supply so as to simultaneously drivethe motion of the X-ray source and the deflection of the resultant imagein the detector. This system thus avoids many of the disadvantagesassociated with mechanically moving the radiation source and detector.However, this system has no provision for consistently maintaining thefocus and energy of the electron beam as the beam is swept over thetarget surface. This causes the X-ray spot to vary in both size andintensity, which seriously limits the resolution achievable with thedevice. The use of electron optics to deflect the electron image alsolimits the detection resolution achievable with this device. Thisproblem becomes especially severe as the image is deflected throughlarge angles. Similarly, accuracy in the positioning of the X-ray spotis lost as the beam is deflected through severe angles. Thesecharacteristics substantially limit the resolution achievable with thistechnique. Furthermore, the technique is practical only for operationwithin a relatively small range of viewing angles, which limits thedesired laminographic blurring effect of unwanted features andconsequently limits the resolution in a direction normal to the plane offocus.

All of the above described laminography systems are directed toperforming body section radiography and, as such, are not designed toproduce high resolution images in rapid succession. Furthermore, suchsystems need not operate in a continuous duty cycle nor in anenvironment compatible with the manufacturing of electronics.

Many of the deficiencies found in presently used electronic inspectionsystems could be overcome with a high resolution, high speedlaminographic inspection system. Such a system would be particularlywell suited for the inspection of electrical connections such as solderjoints in electronic assemblies. A high resolution laminograph of asolder joint should be capable of unambiguously revealing features inthe solder joint which are indicative of the joint quality.Unfortunately, even though many attempts have been made to utilizelaminographic techniques in industrial inspection environments, priorsystems have consistently fallen short of optimum performance because ofpoor image resolution or prohibitively long inspection times, or both.Techniques previously used to improve resolution invariably resulted inlong inspection times. Likewise, techniques previously used to decreaseinspection time have generally sacrificed image resolution. A need thusexists for a high speed, high resolution industrial laminography systemcapable of inspecting electronics in industrial environments.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method for theinspection of electrical connections between electronic components whichare mounted on printed circuit boards. The invention producescross-sectional images of the connections which are then analyzed by acomputer aided image analysis system. The cross-sectional images areautomatically analyzed to identify and locate defects in the connectionsand to determine process characteristics of the connections. A report ofthe image analysis indicating the location and type of defect or processcharacteristic is prepared and presented to the user.

More specifically, the present invention provides an automatedlaminography inspection system for solder joints on printed wiring boardassemblies and other electronic devices and assemblies. A central dataprocessing and control unit positions the item to be inspected, controlsthe formation of the laminographic images, analyzes the image data,makes decisions regarding the characteristics and acceptability of theitem being inspected based on analysis of the image data andcommunicates the results of the inspection process to the user.

The inspection system of the present invention has several significantadvantages over alternative systems and methods. Thus, the presentinvention's performance is superior to these alternate systems andmethods due in part to the high resolution of the images, thecross-sectional format of the images, and the automated rapidacquisition and analysis of the images.

The invention advantageously utilizes the techniques of X-raylaminography to acquire the high resolution cross-sectional images. Theinvention employs circular motions of a radiation source and detector inorder to optimize the laminographic blurring of artifacts withoutresorting to unnecessarily complex motions. Precision circular rotationof the radiation source is effected by causing the electron beam withina stationary X-ray tube to circumscribe a circular path on the anode ofthe X-ray tube, thus eliminating moving parts associated with rotationof the radiation source. Rotation of the detector is the only mechanicalmotion required to generate the laminographic image. A calibratedfeedback system further improves the precision of the system bycompensating for inaccuracies in the mechanical components of the systemwhich affect the alignment and synchronization of the rotating X-raysource and detector during formation of the laminographic images. Thefeedback system coordinates the detector location with the X-ray sourcelocation thus ensuring the continuous and accurate alignment of thesource and detector during acquisition of the images.

The high resolution, high speed laminography inspection system of thepresent invention produces high resolution cross-sectional images ofsolder connections, electronic devices and other assemblies whilemaintaining high inspection rates in the following manner. The rotatingX-ray source and detector produce a rotating X-ray shadowgraph imagewhich impinges upon a fluorescent screen detector that converts theX-ray image into a visible light image. Rotation of the X-ray source isaccomplished electronically, thus eliminating inaccurate and complicatedmechanical mechanisms. The fluorescent screen is carried on a turntablepositioned opposite the rotating X-ray source. The screen rotates in aplane which is parallel to the plane defined by the locus of therotating X-ray source and rotates about a common axis of rotation withthe X-ray source. Also mounted on the turntable is an optical derotationassembly comprising two mirrors which allows the image on thefluorescent screen to be viewed by a stationary camera. Thus the onlymechanical motion required to form a cross-sectional image is therotation of the turntable, which can be rotated at a constant speed,making the mechanical aspects of the system quite simple.

Image resolution is further improved by using a microfocus X-ray sourcein an arrangement which provides geometric magnification of the objectbeing inspected.

Precise alignment of the source and detector also contributes to theproduction of high resolution images and is maintained by a feedbacksystem. The feedback system maintains precise alignment of the rotatingsource spot and fluorescent screen by driving the deflection circuitryof the electron beam within the X-ray tube in synchronization with theposition of the rotating turntable. This feedback technique allows forgreater accuracy than in prior laminography systems by storing in amemory a look-up table of coordinates that indicate the precise signalsto be issued to the deflection circuitry of the X-ray source based uponthe actual position of the turntable as determined by a precisionposition encoder. The feedback system accepts from the position encoderinput data indicating the position of the turntable, retrieves thecorresponding coordinates from the look-up table and drives thedeflection circuitry on the X-ray tube accordingly. The alignment of thesource spot and turntable is periodically calibrated in a procedure thatgenerates an appropriate look-up table of coordinates. Thus theprecision of the laminography system is maintained in spite of minorinaccuracies and variations in the speed of rotation of the turntable,the alignment of the turntable, the shape of the target anode and othercritical parameters determining the inspection geometry.

The printed wiring board, or other object to be inspected, is supportedon a mechanical handling system that can be operated automatically undercomputer control to move the object in a fashion that sequentiallybrings the desired portions of the object into view.

The high resolution cross-sectional image of a solder joint acquired bythe X-ray laminography system is analyzed automatically. A powerfulcomputer system utilizes parallel processing to efficiently andautomatically control the acquisition of a cross-sectional image of thesolder joint, measure characteristics of the image, correlate thecharacteristics with specific types of solder defects and make decisionsregarding the acceptability of the item's quality accordingly. Theresults of the image analysis are communicated to the user in any of avariety of output formats.

These and other characteristics of the present invention will becomeapparent through reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a laminography systemillustrating the principles of the technique.

FIG. 2a shows an object having an arrow, a circle and a cross embeddedin the object at three different planar locations.

FIG. 2b shows a laminograph of the object in FIG. 2a focused on theplane containing the arrow.

FIG. 2c shows a laminograph of the object in FIG. 2a focused on theplane containing the circle.

FIG. 2d shows a laminograph of the object in FIG. 2a focused on theplane containing the cross.

FIG. 2e shows a conventional, two-dimensional X-ray projection image ofthe object in FIG. 2a.

FIG. 3a is a diagrammatic cross-sectional view of a first preferredembodiment of the image forming apparatus of the invention, showing howthe laminographic image is formed and viewed by a camera.

FIG. 3b shows a top view enlargement of an inspection region shown inFIG. 3a.

FIG. 3c is a perspective view of the embodiment of the invention shownin FIG. 3a.

FIG. 4 shows details of an X-ray tube having a rotating spot source ofX-rays for use in the preferred embodiment.

FIG. 5 is a cross-sectional view of the target anode of the X-ray tubeshown in FIG. 4.

FIG. 6 is a cross-sectional view of the rotating X-ray detector andcamera system.

FIG. 7 is a schematic diagram illustrating the calibration procedure forsynchronizing the X-ray source and detector positions.

FIG. 8 is a schematic block diagram for the feedback control system usedfor the synchronization of the X-ray source and detector motions.

FIG. 9a illustrates a test fixture for use in the calibration procedureshown in FIG. 7.

FIG. 9b shows an X-ray image of the test fixture of FIG. 9a.

FIG. 10a is a flowchart of a procedure used to calibrate thesynchronization of the X-ray source and detector positions.

FIG. 10b is a continuation of the flowchart in FIG. 10a.

FIG. 11 is a block diagram of the computer control and analysis system.

FIG. 12 is a schematic flowchart of the operation of the master controlcomputer, showing the automated sequence of operations.

FIG. 13 is a diagram of the timing cycle for the coordinated motion ofthe circuit board and the acquisition of multiple field of view images.

FIG. 14 is an example of an Inspection Report generated by theinvention.

FIG. 15 shows a typical circuit upon which are located multipleelectronic devices interconnected by multiple solder connections.

FIG. 16 shows a typical Leadless Chip Carrier device in position formounting to a circuit board.

FIG. 17 shows examples of good and defective solder connections formedbetween an electronic device and a circuit board.

FIG. 18 shows a cross-sectional image of the solder connections in FIG.17.

FIG. 19 illustrates the procedure for automatically locating andidentifying a solder bridging type defect.

FIG. 20a is a flowchart illustrating the process for automaticallylocating and identifying a solder bridging defect.

FIG. 20b is a continuation of the flowchart in FIG. 20a.

FIG. 21 illustrates the procedure for automatically locating andidentifying a solder connection having insufficient solder.

FIG. 22 is a cross-sectional view of a typical good solder connectionillustrating three regions of the connection.

FIG. 23a is a graphical representation of the image intensity versussolder thickness for a cross-sectional image of solder material.

FIG. 23b shows a calibration step wedge used for calibrating the imageintensity versus thickness relationship.

FIG. 23c is a graphical representation of the image intensity versusthickness relationship for the calibration step wedge shown in FIG. 23b.

FIG. 24 is a flowchart illustrating the process for automaticallylocating and identifying a solder connection having missing orinsufficient solder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used throughout, the term "radiation" refers to electromagneticradiation, including but not limited to the X-ray, gamma and ultravioletportions of the electromagnetic radiation spectrum.

FIG. 1 shows a schematic representation of the laminographic geometryused in the present invention. An object 10 under examination, forexample, a circuit board, is held in a stationary position with respectto a source of X-rays 20 and an X-ray detector 30. Synchronous rotationof the X-ray source 20 and detector 30 about a common axis 40 causes anX-ray image of the plane 60 within the object 10 to be formed on thedetector 30. The image plane 60 is substantially parallel to the planes62 and 64 defined by the rotation of the source 20 and detector 30,respectively. The image plane 60 is located at the intersection 70 of acentral ray 50 from the X-ray source 20 and the common axis of rotation40. This point of intersection 70 acts as a fulcrum for the central ray50, thus causing an in-focus cross-sectional X-ray image of the object10 at the plane 60 to be formed on detector 30 as the source anddetector synchronously rotate about the intersection point 70. Structurewithin the object 10 which lies outside of plane 60 forms a blurredX-ray image on detector 30.

The laminographic geometry shown in FIG. 1 is the geometry preferred forthe present invention. However, it is not necessary that the axis ofrotation of the radiation source 20 and the axis of rotation of thedetector 30 be coaxial. The conditions of laminography are satisfied anda cross-sectional image of the layer 60 will be produced as long as theplanes of rotation 62 and 64 are mutually parallel, and the axes ofrotation of the source and the detector are mutually parallel and fixedin relationship to each other. This reduces the number of contraintsupon the mechanical alignment of the apparatus of the present invention.

FIGS. 2a-2e show laminographs produced by the above describedlaminographic technique. The object 10 shown in FIG. 2a has testpatterns in the shape of an arrow 81, a circle 82 and cross 83 embeddedwithin the object 10 in three different planes 60a, 60b and 60c,respectively.

FIG. 2b shows a typical laminograph of object 10 formed on detector 30when the point of intersection 70 lies in plane 60a of FIG. 2a. Theimage 100 of arrow 81 is in sharp focus, while the images of otherfeatures within the object 10, such as the circle 82 and cross 83 form ablurred region 102 which does not greatly obscure the arrow image 100.

Similarly, when the point of intersection 70 lies in plane 60b, theimage 110 of the circle 82 is in sharp focus as seen in FIG. 2c. Thearrow 81 and cross 83 form a blurred region 112.

FIG. 2d shows a sharp image 120 formed of the cross 83 when the point ofintersection 70 lies in plane 60c. The arrow 81 and circle 82 formblurred region 122.

For comparison, FIG. 2e shows an X-ray shadow image of object 10 formedby conventional projection radiography techniques. This techniqueproduces sharp images 130, 132 and 134 of the arrow 81, circle 82 andcross 83, respectively, which overlap one another. FIG. 2e vividlyillustrates how multiple characteristics contained within the object 10may create multiple overshadowing features in the X-ray image whichobscure individual features of the image.

FIG. 3a illustrates a schematic diagram of a preferred embodiment of theinvention. In this preferred embodiment, an object under inspection is aprinted circuit board 210 having multiple electronic components 212mounted on the board 210 and electrically interconnected via electricalconnections 214 (see FIG. 3b). Typically, the electrical connections 214are formed of solder. However, various other techniques for making theelectrical connections 214 are well know in the art and even though theinvention will be described in terms of solder joints, it will beunderstood that other types of electrical connections 214 including, butnot limited to, conductive epoxy, mechanical, tungsten and eutecticbonds may be inspected utilizing the invention. FIG. 3b, which is a topview enlargement of a region 283 of the circuit board 210, more clearlyshows the components 212 and solder joints 214.

The invention acquires cross-sectional images of the solder joints 214using the previously described laminographic method or other methodscapable of producing equivalent cross-sectional images, such as standardcomputerized tomography techniques. The cross-sectional images of thesolder joints 214 are automatically evaluated to determine theirquality. Based on the evaluation, a report of the solder joint qualityis presented to the user.

The invention, as shown in FIG. 3a, comprises an X-ray tube 200 which ispositioned adjacent printed circuit board 210. The circuit board 210 issupported by a fixture 220. The fixture 220 is attached to a positioningtable 230 which is capable of moving the fixture 220 and board 210 alongthree mutually perpendicular axes, X, Y and Z. A rotating X-ray detector240 comprising a fluorescent screen 250, a first mirror 252, a secondmirror 254 and a turntable 256 is positioned adjacent the circuit board210 on the side opposite the X-ray tube 200. A camera 258 is positionedopposite mirror 252 for viewing images reflected into the mirrors 252,254 from fluorescent screen 250. A feedback system 260 has an inputconnection 262 from a sensor 263 which detects the angular position ofthe turntable 256 and an output connection 264 to X and Y deflectioncoils 281 on X-ray tube 200. A position encoder 265 is attached toturntable 256. The position sensor 263 is mounted adjacent encoder 265in a fixed position relative to the axis of rotation 40. The camera 258is connected to a master computer 270 via an input line 276. The mastercomputer 270 is connected to a high speed image analysis computer 272.Data is transferred between the master computer 270 and the imageanalysis computer 272 via data bus 274. An output line 278 from mastercomputer 270 connects the master computer to positioning table 230.

A perspective view of the invention is shown in FIG. 3c. In addition tothe X-ray tube 200, circuit board 210, fluorescent screen 250, turntable256, camera 258, positioning table 230 and computers 270, 272 shown inFIG. 3a, a granite support table 290, a load/unload port 292 and anoperator station 294 are shown. The granite table 290 provides a rigid,vibration free platform for structurally integrating the majorfunctional elements of the invention, including but not limited to theX-ray tube 200, positioning table 230 and turntable 256. The load/unloadport 292 provides a means for inserting and removing circuit boards 210from the machine. The operator station 294 provides an input/outputcapability for controlling the functions of the invention as well as forcommunication of inspection data to an operator.

In operation of the invention as shown in FIGS. 3a and 3c, highresolution, cross-sectional X-ray images of the solder joints 214connecting components 212 on circuit board 210 are acquired using theX-ray laminographic method previously described in reference to FIGS. 1and 2. Specifically, X-ray tube 200, as shown in FIG. 3a, comprises arotating electron beam spot 285 which produces a rotating source 280 ofX-rays 282. The X-ray beam 282 illuminates a region 283 of circuit board210 including the solder joints 214 located within region 283. X-rays284 which penetrate the solder joints 214, components 212 and board 210are intercepted by the rotating fluorescent screen 250.

Dynamic alignment of the position of the X-ray source 280 with theposition of rotating X-ray detector 240 is precisely controlled byfeedback system 260. The feedback system correlates the position of therotating turntable 256 with calibrated X and Y deflection values storedin a look-up table (LUT). Drive signals proportional to the calibrated Xand Y deflection values are transmitted to the steering coils 281 on theX-ray tube 200. In response to these drive signals, steering coils 281deflect electron beam 285 to locations on an annular shaped target anode287 such that the position of the X-ray source spot 280 rotates insynchronization with the rotation of detector 240 in the mannerpreviously discussed in connection with FIG. 1.

X-rays 284 which penetrate the board 210 and strike fluorescent screen250 are converted to visible light 286, thus creating a visible image ofa single plane within the region 283 of the circuit board 210. Thevisible light 286 is reflected by mirrors 252 and 254 into camera 258.Camera 258 typically comprises a low light level closed circuit TV(CCTV) camera which transmits electronic video signals corresponding tothe X-ray and visible images to the master computer 270 via line 276.The electronic video format image is transferred to the high speed imageanalysis computer 272 via line 274. The image analysis computer 272analyzes and interprets the image to determine the quality of the solderjoints 214.

Master computer 270 also controls the movement of positioning table 230and thus circuit board 210 so that different regions of circuit board210 may be automatically positioned within inspection region 283.

ROTATING X-RAY SOURCE

FIG. 4 illustrates an X-ray tube 200 capable of providing the rotatingbeam of X-rays 282 for producing high resolution laminographs of circuitboards. The tube 200 comprises an electron gun 310 mounted adjacent ahigh voltage electrode section 320. Focus coils 330 and steering coils281 are positioned intermediate the electrode section 320 and theannular shaped target anode 287. An electron beam stop 360 and X-raywindow 370 are mounted within the central area defined by the annularshaped anode 287. A vacuum envelope 380 encloses the evacuated portionsof the X-ray tube assembly 200.

In operation, electron gun 310 emits an electron beam 285 into the highvoltage electrode section 320. A high DC voltage is applied between theelectron gun 310 and target anode 287 to accelerate and guide electronbeam 285 toward a collision with the anode 287. Portions of the highvoltage signal are applied to electrodes 322 which guide, accelerate,and shape electron beam 285. In a preferred embodiment, the high voltagesignal is approximately 160 kilovolts and is capable of providingapproximately 7.5 microamps of current through electron beam 285 toanode 287. Preferably, the high voltage signal is maintained constant towithin an accuracy of approximately 0.01%. It will be understood thatthese values are exemplary and that other voltages, currents, andaccuracies may also be used.

After traversing electrode section 320, the electron beam 285 enters aregion of the tube wherein the shape and direction of the electron beamare affected by the focus coils 330 and steering coils 281. In apreferred embodiment, the coils 330 and 281 produce electromagneticfields which interact with the electron beam 285 to focus as well asdirect the electron beam 285 toward specific locations on the anode 287.The X-ray source 280 coincides with these specific locations from whichthe X-ray beam 282 is emitted. In this manner, an extremely small,approximately 20-micron diameter electron beam spot is formed on theanode 287 at these locations. As is well known in the field ofradiography, the size of this spot plays a very important role indetermining the overall resolution of the X-ray images obtained from thesource of X-rays 280.

The steering coils 281, in combination with the annular shaped anode 287enable the X-ray tube 200 to provide X-rays from source 280 wherein thelocation of the source 280 moves in a circular pattern around the anode.The circular pattern is centered about a fulcrum point 402 locatedwithin a cross-sectional image plane 403 of object 410.

Specifically, the steering coils 281 are capable of directing theelectron beam 285 toward any desired portion of an inner surface 354 ofanode 287. By driving the electromagnetic coils 281 with appropriatelysynchronized X and Y drive signals, the electron beam 285 can be steeredtoward the anode 287 such that the beam inscribes a circular path alongthe inner surface 354 of the anode 287.

In a preferred embodiment, the steering coils 281 comprise separate Xand Y electromagnetic coils which deflect electron beam 285 in the X andY directions respectively. Electrical current flowing in the coils 281creates magnetic fields which interact with the electron beam 285causing the beam to be deflected. These coils 281 are similar instructure and function to the yoke coils found in cathode ray tubes(CRT). It will be understood, however, that electrostatic deflectiontechniques could also be used to deflect the electron beam 285.

The surface 354 upon which the electron beam 285 strikes the anode 287is shaped so that a central X-ray 392 of X-ray beam 282 originates atsource location 280 and is directed toward the fulcrum point 402. Thus,as the electron beam 285 circumscribes a circular path along the surface354, the central beam 392 is always directed toward the same location402.

The material forming the surface 354 of the anode 287 is selected sothat the radiation produced when electron beam 285 strikes the surface354 has the desired energy characteristics. The radiation produced bybombarding a target material with an accelerated electron beam is knownas Bremsstrahlung radiation. The characteristics of Bremsstrahlungradiation are determined primarily by the energy of the electron beamand the material composition of the target into which the electron beamis directed. In a preferred embodiment, the surface 354 which isbombarded by electron beam 285 is covered with a layer of tungstenmetal.

The substrate 356 on which the tungsten surface 354 is placed may becopper or other suitable metal. A material with a high heatconductivity, such as copper, is particularly well suited for thisapplication since significant heating of the target anode 287 occurswhen the energy of the electron beam 285 is deposited in the anode. Thecopper substrate 356 provides a very efficient heat conductor forremoving this heat from the locations 280 where electron beam 285collides with the anode 287.

The radiation beam 282 produced in the collision of electron beam 285with tungsten layer 354 exits the tube 200 through a window 370. Thewindow 370 forms a portion of the vacuum envelope of the tube 200 inwhich the electron beam 285 propagates which allows the X-rays producedwithin the tube at the surface 354 to exit the vacuum portion of thetube with minimal loss of intensity and energy. Titanium is commonlyused to form X-ray windows for X-ray tubes and is preferred in thisembodiment for window 370. However, it will be understood that othermaterials could also be used to form the window 370.

During the X-ray inspection of a circuit board or other object 410, itis often advantageous to turn off the X-rays while the circuit board isbeing moved so that different regions of the board fall within theinspection area 400. It is desirable that the X-rays be turned on andoff as rapidly as possible. Additionally, it is desirable to perform theON/OFF cycling so that the X-rays produced during all of the ON portionsof the cycles have substantially identical energy, intensity and opticalcharacteristics. X-ray tube 200 accomplishes this rapid ON/OFFstabilized cycling of the X-rays by directing the electron beam 285 intothe beam stop 360. This diversion of electron beam 285 prevents X-raysfrom exiting the window 370. Thus, radiation production directed towardthe object 410 is stopped, i.e. turned off, while the object is beingrepositioned. Steering coils 281 provide a fast means for accomplishingthis deflection of electron beam 285 into the beam stop 360. This methodof turning off the X-rays enables the electron beam 285 and all otherfunctions of the X-ray tube which affect the X-ray beam 282characteristics to be left undisturbed during the ON/OFF cycling.Therefore, when the electron beam 285 is redirected to the anode 287 forthe ON portion of the cycles, the characteristics of X-ray beam 282 aresubstantially unchanged from previous ON cycles.

The beam stop 360 is formed of a material which is highly attenuative ofX-rays, for example, lead or copper. The thickness, location, and shapeof the beam stop 360 are selected to prevent X-rays from exiting thetube 200 via the window 370 when the beam is directed into the beamstop. These parameters are easily determined by one skilled in the artof X-ray tube design.

An enlarged cross-sectional view of anode 287 is shown in FIG. 5. Inthis preferred embodiment, the annular target surface 354 comprises aportion of a cone which is symmetric about an axis 404. The target anode287 is mounted to the tube 200 such that the axis 404 of the conecoincides with the central Z axis of the tube 200. Thus, when theelectron beam 285 is steered in a circular oscillation with the radiusr₁ shown as 406, the effect is that of a moving spot source of radiation280 having energy, intensity, and focus characteristics equivalent toconventional stationary radiation sources. It will be understood thatother shapes for the anode 287 may be used which will produce equivalentresults.

The X-ray source 200 thus provides a source of X-rays suitable formaking high resolution X-ray images even when used in a geometry whichmagnifies the images. One such magnifying geometry is one wherein theobject being imaged is located between an X-ray source and an X-raydetector such that the distance from the object to the X-ray source isless than the distance from the object to the X-ray detector.Additionally, source 200 has the capability of moving this source ofX-rays in a circular pattern suitable for making laminographs. Thiscircular motion is accomplished without sacrificing image resolution orspeed of acquisition. Since the rotation of the radiation source isaccomplished electronically, no moving parts are needed, thuseliminating vibrations and other undesirable characteristics ofmechanical systems. An X-ray source having the above describedcharacteristics is available from Kevex Corporation as Model No. KM160R.Other electrically steered moving X-ray sources are described in U.S.Pat. No. 4,075,489 entitled "Method and Apparatus Involving theGeneration of X-Rays"; U.S. Pat. No. 4,352,021 entitled "X-RayTransmission Scanning System and Method and Electron Beam X-Ray ScanTube for Use Therewith"; and U.S. Pat. No. 2,319,350 entitled "X-RayTube and Apparatus." These patents are hereby incorporated herein byreference.

ROTATING X-RAY DETECTOR

Shown in FIG. 6 is an embodiment of the rotating X-ray detector system240 discussed previously in connection with FIG. 3a and used inconjunction with the rotating X-ray source 280 to acquirecross-sectional images of an object 630. As shown in FIG. 6, an X-rayimage of the object 630 is formed on the rotating fluorescent screen 250by X-ray beam 284. Screen 250 converts these X-rays to optical signals286 for detection by conventional optical devices. In the preferredembodiment, the optical signals 286 from rotating fluorescent screen 250are detected by the closed circuit TV (CCTV) camera 258. Camera 258converts the optical signals 286 to electrical signals for furtherprocessing by computer systems 270 and 272. The optical image formed onthe screen 250 rotates with the screen. In order to eliminate the needfor mechanical motion of the CCTV camera 258 which views the rotatingoptical image, the optical image is derotated within the rotatingdetector 240 by optical mirrors 252 and 254 so that the rotating opticalimages formed on the rotating screen 250 appear stationary as viewed bythe camera.

The rotating X-ray detector 240 comprises the turntable 256 rotatablymounted about the axis 404 by a bearing 700. It is noted that the axis404 is nominally the same axis about which the source of rotating X-rays280 revolves. The fluorescent screen 250 is attached to the top ofturntable 256. The two mirrors 252 and 254 are mounted within turntable256 parallel to one another and at an angle of 45° with respect to axis404. The mirror 252 is mounted in the center of turntable 256 so that itintersects the axis 404 near the center of the mirror. The mirror 254 ismounted within turntable 256 so that it faces both the first mirror 252and the fluorescent screen 250. Fluorescent screen 250 and mirrors 252and 254 are attached to turntable 256 so that the turntable, mirrors andscreen rotate about axis 404 as a single unit. This arrangement ofmirrors, turntable, and screen forms an optical derotation assembly foroptical images formed on the screen 250 when the detector 240 is rotatedabout the axis 404.

An X-ray shadow image of the object 630 is formed on the fluorescentscreen 250 when the X-ray beam 284 strikes the screen. The fluorescentscreen 250 functions as an X-ray to optical converter. For example, whenX-rays 284 strike the surface 651 of the screen 250 which faces theX-ray source 280, visible light 286 is emitted from the screen surface652 opposite the X-ray source 280. Optical signals 286 emitted from thefluorescent screen surface 652 are reflected by the two parallel mirrors252 and 254 into a lens 699 attached to the closed circuit TV camera258.

The fluorescent screen 250 is mechanically rotated at a uniform angularvelocity about the axis 404 in plane 64 which is substantially parallelto plane 62 defined by the circular motion of the moving spot source ofradiation 280. The mirrors 252, 254 reflect the optical image from therotating fluorescent screen into the stationary camera system 258through the lens 699 so that the rotation of the image in the plane 64is not apparent to the camera 258. This mirror arrangement has beenpreviously described in U.S. Pat. No. 2,998,511 entitled "Tomoscope."

As a result of the fixed mounting of the fluorescent screen 250 to therotating turntable 256, successive images of object 630 formed on thescreen have different orientations with respect to the screen as ittraverses its circular path about the axis 404. Thus, in order to avoidblurring of the image caused by the movement of the image with respectto the screen, it is desirable that the fluorescence of a point on thescreen surface be suppressed abruptly after that point is no longer hitby an X-ray. In a preferred embodiment, the fluorescent screen 250comprises praseodymium-doped gadolinium oxysulfide, Gd₂ O₂ SiPr.Praseodymium-doped gadolinium oxysulfide is a scintillation materialwhich is "fast" enough to prevent blurring due to motion of the imagewith respect to the screen and also provides sufficient light output fordetection by the camera system 258.

Alternatively, "slower" screens may be used, for example, a screen whichcomprises a cadmium tungstate scintillation material. However, rotatablymounted to the turntable 256 such that an image of object 630 formed onthe screen remains stationary with respect to the screen. Such a motionmay be accomplished, for example, by a set of gears which superimposes acircular motion of the screen with respect to the turntable insynchronization with the rotation of the turntable about the axis 404.

An alternative embodiment (not shown) for the rotating X-ray detector240 which forms an optical derotation assembly replaces the two flatmirrors 252,254 with a suitably bent bundle of image conductors, e.g.,optical fibers, which are coupled to the fluorescent screen 250 androtate in unison with the screen. The image conductors transmit theimage from the fluorescent screen 250 to a position centered on the axisof rotation 404, to the same effect as the two parallel mirrors shown inFIGS. 3a and 6. These image conductors may comprise optical fibers,electron conductors or equivalent devices.

CROSS-SECTIONAL IMAGE FORMATION

As previously discussed, a cross-sectional image of object 630 is formedon screen 250 as the screen 250 and X-ray source 280 synchronouslyrotate about axis 404. The blurring effects of the laminography methodand image resolution are maximized when the cross-sectional image isacquired during a full rotation of the screen 250 and source 280 aboutthe axis 404. The camera system 258 detects the development of thecross-sectional image on the fluorescent screen 250 by means of theoptical derotation assembly comprising mirrors 252 and 254.

Since the fluorescent screen 250 may not emit high intensity opticalsignals, it is often advantageous to detect the optical signals 286 witha high sensitivity, low light level device. Use of a low light leveldetection device thus improves the detected image quality by detecting alarger portion of the optical signals 286 emitted from the fluorescentscreen 250 during a single rotation of the screen. Many low light levelcamera systems incorporate an image intensifier as part of the camerasystem to improve the low light level sensitivity. One particular systemis known as a silicon intensified target (SIT) camera and is capable ofdetecting extremely low levels of light. SIT camera systems are wellknown and readily available. A preferred embodiment of the presentinvention utilizes a SIT camera system which is based upon the RCA ModelNo. 4804BHP2-12 SIT tube.

In a preferred embodiment, one cross-sectional image is acquired inapproximately 0.1 seconds during the rotation of the fluorescent screen250 about the axis 404 at the rate of approximately 600 revolutions perminute. During one complete revolution, three video frames, each framehaving a duration of 1/30 of a second, are collected by the camera 258.The three video frames are communicated from the camera 258 to mastercomputer 270 (shown in FIG. 3a) where the three frames are averagedtogether, thus forming a digital representation of the cross-sectionalimage of the object 630 formed on the fluorescent screen 250 during asingle rotation of the screen 250 about the axis 404. Alternatively, thecamera 258 may be connected to a CRT, so that the cross-sectional imagecan be viewed directly.

SOURCE/DETECTOR SYNCHRONIZATION

Formation of a high resolution laminographic cross-sectional imagedepends upon the precise alignment and synchronization of the circularmotions of the radiation source 280 and detector screen 250. Asillustrated in FIG. 7, proper alignment and synchronization are achievedwhen central X-ray 392 from source 280 passes through a fixed point 780lying on the axis 404, such that the central X-ray 392 is alwaysdirected to a single point 880 on the surface of detector screen 250.For the configuration shown in FIG. 7, this is clearly achieved when theangular positions of the source and detector screen, relative to a fixedreference position, are separated by 180°.

The preferred alignment and synchronization of the source 280 anddetector screen 250 are maintained by the feedback system 260 shown inFIG. 3a. The position of the rotating turntable 256, upon which theX-ray detector screen 250 is mounted, is monitored by the sensor 263.The turntable position is communicated to the feedback system 260, whichsupplies drive signals corresponding to the position of the turntable tothe electron beam deflection coils 281. The drive signals control theposition of the X-ray source 280 such that the source 280 and screen 250are always in alignment as the turntable rotates about the axis 404. Inthis manner, the feedback system maintains the precision geometrynecessary for the production of high resolution cross-sectional images.This system compensates for alignment inaccuracies of the X-ray tube 200and rotating X-ray detector 240; machining, mounting and fabricationinaccuracies and defects of the target anode 287 and its surface coating354; aberrations, such as astigmatism, in the electron beam 285 paththrough the X-ray tube; and variations in the rotational velocity of therotating turntable during image formation.

A detailed block diagram of feedback system 260 is shown in FIG. 8.Feedback system 260 comprises X and Y look-up tables (LUTs) 720X and720Y, respectively, X and Y digital-to-analog converters (DACs) 723X and723Y, respectively, and X and Y coil drivers 724X and 724Y,respectively. The LUTs 720X and 720Y are preferably solid state, digitalrandom access memories (RAM). The feedback system links the rotatingX-ray detector 240 to the X-ray tube deflection coils 281 under controlof the master computer system 270.

As rotating X-ray detector 240 revolves about axis 404, the positionsensor 263 detects the angular position of the detector 240 from theposition encoder 265. The detected angular position is converted to Xand Y address signals which correspond to the angular position of thedetector. The address signals are communicated to the X and Y LUTs 720X,720Y via a communication line 721. By means of a source/detectoralignment calibration procedure, X and Y calibration data are determinedand stored in the X and Y LUTs for each angular position of thedetector. Thus, there exists a one to one correspondence between the Xand Y addresses from the encoder and the X and Y calibration data in theLUT's. The X and Y calibration data are retrieved from the LUT's in theform of electronic digital signals. The electronic digital signals aretransmitted from the X and Y LUTs to the X and Y DACs 723X and 723Y,respectively, via communication lines 722X and 722Y. The DACs convertthe digital signals into analog electrical signals which travel vialines 725X and 725Y to the coil drivers 724X and 724Y. The coil driversamplify their respective analog input signals and apply resulting outputsignals via lines 726X and 726Y to the coils 281X and 281Y,respectively, to achieve the precise deflection of the electron beam 285required for proper alignment of the source and detector. The electronbeam is deflected through interaction with magnetic fields generated bythe application of the output signals to the coils 281. As the electronbeam traverses the magnetic fields, it is deflected, thus moving theposition of the X-ray source spot 280 on the anode 287. The distance thespot moves is proportional to the magnitude of the drive signals asdetermined by the calibration data.

The LUT calibration data are determined using the calibrationconfiguration schematically illustrated in FIG. 7. A test pattern 730 ispositioned between the X-ray source 280 and detector screen 250 suchthat test pattern 730 intersects the axis 404 at location 780. Aconventional X-ray shadowgraph image 830 of test pattern 730 is formedon screen 250. The optical representation of X-ray image 830 on thescreen is viewed by the camera 258 (See FIG. 3a). An electricalrepresentation of the optical image is output from camera 258 by meansof electrical signals on line 276 to master computer 270 and imageanalysis computer 272. The electrical signals on line 276 are digitizedby computer 270 and stored in the memory of computer 270 in digitalformat.

A preferred embodiment of test pattern 730, shown in FIG. 9a, comprisesa foundation 732 of material such as plastic, which is relativelytransparent to X-rays. Foundation 732 is approximately 0.5×0.5 inch inlength and width, and approximately 0.1 inch thick. At a center location780 on foundation 732 are mounted three pieces of 0.001 inch diametertungsten wire 781a, 781b and 781c, oriented such that wire 781cintersects the center location 780. Wires 781a and 781b are mounted tofoundation 732 so that they are on opposite sides of wire 781c, and sothat a line connecting wires 781a and 781b also intersects centerlocation 780. Thus, wires 781a, 781b and 781c form a fiducial crosshair781 having its center at location 780. Mounted around the crosshair 781are eight markers 782 made of lead or other X-ray opaque material. Leadmarkers 782a through 782d are approximately 0.0625 inch square and 0.004inch thick, and are located near the four corners of the foundation 732.Lead markers 782e through 782h are approximately 0.0625 inch square and0.008 inch thick, and are positioned intermediate the markers 782athrough 782d.

A representation of a typical X-ray shadowgraph image 830 of testpattern 730 is shown in FIG. 9b. Lead markers 782a through 782h formimage regions 882a through 882h, respectively, of image 830. The center780 of test pattern 730 is represented by image center 880. Likewise,tungsten wires 781a through 781c form image regions 881a through 881c,respectively.

A portion of image 830 represented by the dotted lines 884a through 884dforms a rectangular region of interest (ROI) 884 which surrounds theimages of lead markers 882 and tungsten wires 881. Region of interest884 is stored in the computer 270 in digital format. As is well known,digitally stored images comprise an array of pixels, each pixelrepresenting a small portion of the image. Specifically, region ofinterest 884 is divided into a pixel grid comprising 512 columns alongborder 884a and 480 rows along border 884b. Each pixel in the grid maybe represented by its corresponding column and row designation. Forexample, the lower left corner 885 of region of interest 884 isrepresented by the pixel (0,0). Similarly, corner 886 is represented bypixel (511,0), corner 887 is represented by pixel (511,479) and corner888 is represented by pixel (0,479). The center location 880 isrepresented by pixel (256,240). In one embodiment, the distance betweencorners 885 and 886 of the image 830 corresponds to approximately 0.400inches on the test pattern 730. Likewise, the distance between corners885 and 888 of the image corresponds to approximately 0.375 inches onthe test pattern.

Determination of the calibration data for the X and Y LUTs is performedeither manually or automatically using the test pattern 730. Referringagain to FIGS. 3a and 7, an initial alignment of the X-ray source 280,test pattern 730, turntable 256 and camera 258 is performed manually.First, the test pattern 730 is positioned so that the center 780intersects the axis 404. The X-ray tube 200, turntable 256 and camera258 are then mechanically aligned so that the test fixture image 830,formed on the screen 250, is continuously within the field of view ofthe camera throughout a complete revolution of the source 280 andturntable 256 about the axis 404. After the system is thus mechanicallyaligned, the turntable 256 is positioned at an initial angular positiondefined as θ=0°. In this initial position, the center pixel (256,240) ofthe digital image detected by the camera and stored in the computercorresponds to a location 880a on the screen 250. The source 280 ispositioned at a location 280a which corresponds to an angular positionof approximately θ=180°, thus placing the test pattern image 830 withinthe field of view of the camera. If the image center 880 of the testpattern image 830 does not fall within the center pixel (256,240), thenthe X and Y deflection values are adjusted to change the position 280aof the source 280, which in turn changes the location of the imagecenter 880 on the screen 250. The deflection values are adjusted untilthe image center 880 is caused to be precisely located at center pixellocation (256,240). These deflection values are then stored in the LUT's720 as the calibration data for the turntable 256 position θ=0°.Turntable 256 and screen 250 are then moved to a new angular positioncorresponding to an angle θ=Δθ. Source 280 is moved to a position 280bcorresponding to an angular position of approximately θ=Δθ+180°, thusplacing the test pattern image 830 within the field of view of thecamera. If the image center 880 of the test pattern image 830 does notfall within the center pixel (256,240), then the X and Y deflectionvalues are adjusted to change the position 280b of the source 280 sothat the image center 880 is again caused to be precisely located atcenter pixel location (256,240). These deflection values are then storedin the LUT's as the calibration data for the turntable 256 positionθ=Δθ°. This procedure for determining the LUT calibration data iscontinued in increments of Δθ° until the source 280 and turntable 256have completed one revolution about the axis 404.

The LUT calibration data determined for positions 280a, 280b, 280c, . .. , 280n of the source are used to determine the formula representing acircle of radius r as a function of angular position θ. The radius r isthe nominal radius of the path followed by the rotating source 280. Thisformula is then used to calculate calibration data for locations of thesource intermediate the locations 280a, 280b, 280c, . . . , 280n.

FIG. 10 illustrates a basic flow chart of the logical sequence of stepsperformed by the calibration procedure to determine the X and Ycalibration data which are stored in the X and Y LUTs 720 forcontrolling the deflection coils 281. First, as previously described,the mechanisms of the invention, as shown in FIG. 3a, including theX-ray tube 200, turntable assembly 256 and XYZ positioning table 230 areassembled and mounted in approximate alignment. Next, test pattern 730is mounted to the XYZ positioning table and moved by the XYZ positioningtable to a position such that the center location 780 of the testpattern 730 coincides with the point 780 represented by the intersectionof the central axis 404 and the nominal center X-ray 392 from the X-raysource 280 (See FIG. 7).

The step of mechanically aligning the X-ray tube 200 and opticalassemblies is represented by an activity block 904 in FIG. 10. Controlis passed from activity block 904 via a path 906 to an activity block908, wherein the test fixture 730 is mounted and aligned on thepositioning table. Control then passes via a path 910 to an activityblock 912 wherein the X-ray source is turned on and the electron beam isdirected to a beam dump location. This allows the X-ray tube tostabilize without subjecting the test pattern and detector to X-rays.Control proceeds via path 914 to an activity block 916 wherein theangular position variable θ and an address indexing variable i areinitialized at θ=0° and i=1, respectively. Control is passed fromactivity block 916 to activity block 920 via a path 918. Activity block920 represents the initialization of the LUTs with initialapproximations given by

    L.sub.xi =A.sub.r sinθ                               (1)

    L.sub.yi =A.sub.r cosθ                               (2)

where A_(r) is proportional to the approximate radius of the rotatingsource 280 and i is the LUT address which contains deflection datacorresponding to the angular position θ. In activity block 924, reachedvia path 922 from block 920, the angular position θ is incremented by anamount Δθ and the index i is incremented by 1. In one preferredembodiment, the angular increment Δθ is approximately 0.022°,corresponding to approximately 16,384 angular positions in onerevolution. In this embodiment, the X and Y LUT's each have at least16384 address locations for the storage of deflection data correspondingto each discrete angular position, and the addressing index i takes onintegral values ranging from 1 to at least 16384. Control then passesvia path 926 to decision block 928. In decision block 928, the value ofθ is checked to see if it is greater than or equal to 360°. If θ is notgreater than or equal to 360°, then control returns to block 920 viapath 930. If θ is greater than or equal to 360°, then control passes viapath 932 to activity block 934. The steps from 920 through 928 form aloop wherein all of the available LUT addresses are loaded with initialdeflection values which will cause the electron beam to circumscribe acircular path upon the anode of the X-ray tube. In the embodiment having16,384 discrete angular positions, the steps 920 through 928 will beexecuted approximately 16,384 times.

Upon completion of the LUT initialization process, control passes viapath 932 to activity block 934, wherein the detector is positioned at aninitial reference location defined as φ=0°. Control is then transferredvia path 936 to activity block 938, wherein the current data(L_(xi),L_(yi)) stored in the LUT's are used to control the rotation ofthe X-ray source. When activity block 938 is entered via path 936, thecurrent data in the LUT's are the initial values calculated inaccordance with equations (1) and (2) and represent an initialapproximation of the final values to be calculated by the belowdescribed calibration procedure.

Determination of the LUT calibration data proceeds via a path 940 toactivity block 942. In block 942, the rotating X-ray source 280 isstopped at the angular position θ which is approximately equal to(φ+180°) , where φ is the angular position of the X-ray detector. Forexample, when the detector is at the initial position φ=0°, then theX-ray detector is positioned at angular position 180° in block 942. Inthe embodiment having 16,384 angular positions and corresponding LUTaddresses, the deflection values stored in LUT memory locationsL_(x8192) and L_(y8192) will produce the deflection of the electron beamto the location on the anode corresponding to an angular position of theX-ray source of 180°.

Subsequent to stopping the rotating X-ray source at angle θ in activityblock 942, control is passed via line 944 to activity block 946. Inactivity block 946, a cross-sectional image 830 of test pattern 730 isobtained and stored in a digital image memory. In a preferredembodiment, the image memory comprises a pixel grid having 512 columnsand 480 rows.

A path 948 transfers control from activity block 946 to an activityblock 950, wherein the pixel(s) (C_(c),R_(c)) containing the location ofthe image center 880 of image 830 are located. C_(c) and R_(c) are thecolumn and row designations respectively, of the image pixel containingthe center of the image, and may be identified manually or automaticallyby means of computer analysis techniques.

The image center pixel position (C_(c),R_(c)) determined in activityblock 950 is transferred to activity block 954 via path 952, wherein therelative offset of the image center from the detector center iscalculated according to the following equations.

    ΔC=256-C.sub.c                                       (3)

    ΔR=240-R.sub.c                                       (4)

ΔC and ΔR represent the distance by which the center of the test patternimage (C_(c),R_(c)) is offset from the center of the digital imagedefined as pixel (256,240).

The ΔC and ΔR values calculated in activity block 954 are transferredvia path 956 to decision block 958, wherein ΔC and ΔR are compared tothe value zero. If ΔC or ΔR is not substantially equal to zero, i.e., iftheir absolute values are not less than some arbitrarily small number,ε, then the test pattern image center is not coincident with the digitalimage center and control is passed via path 960 to activity block 962where the LUT calibration data are adjusted accordingly.

In activity block 962, the LUT calibration data L_(xi) and L_(yi) areadjusted in accordance with the following equations.

    L.sub.xi '=L.sub.xi +f(ΔC,ΔR)                  (5)

    L.sub.yi '=L.sub.yi +g(ΔC,ΔR)                  (6)

Mathematical functions f(ΔC,ΔR) and g(ΔC,ΔR) are used to calculate themagnitude of adjustments for the LUT values L_(xi) and L_(yi)respectively, which will reduce the centering errors ΔC and ΔR. Thevalues L_(xi) and L_(yi) in the LUT's are replaced with the adjustedvalues L_(xi) ' and L_(yi) ' respectively. These adjusted LUT values aretransmitted to the activity block 938 via line 964 and a first loopcomprising the steps 938, 942, 946, 950, 954, 958, and 962 isre-executed until the image center is substantially coincident with thedigital image center. When the image is centered, ΔC and ΔR aresubstantially equal to zero and control passes from decision block 958via path 960 to activity block 968.

In block 968, the detector position is incremented by the amount Δφ tothe next angular position (φ+Δφ). The new angular position of thedetector is passed via path 970 to decision block 972 to determine ifthe new angle φ is greater than or equal to 360°. If φ is less than360°, then control passes via path 974 to activity block 938. A secondloop comprising the first loop and additional steps 968 and 972 isre-executed until the detector has completed one revolution, i.e., whenφ is greater than or equal to 360°.

In a preferred embodiment, the angular increment Δφ is selected to besubstantially larger than the angular increment Δθ between successiveentries in the LUT's so that a calibration for a complete revolution canbe calculated in a short period of time. For example, if the incrementΔφ is equal to 10°, then a complete revolution can be calculated with 35executions of the second loop. The remaining LUT values corresponding topositions intermediate the 36 calculated positions are determined byinterpolating between the adjacent calculated values as indicated inactivity block 978. Control is then passed to activity block 982 viapath 980 for random testing of the centering of the image.

In activity block 982, random angular positions are selected where theaccuracy of the centering is determined. A centering error, ERR, iscalculated which reflects the cumulative error of all of the selectedpositions. The centering error value is passed via path 984 to decisionblock 986 wherein the value is compared to zero or some otherpredetermined value. If ERR is not substantially zero, then controlpasses via path 988 to activity block 990.

In activity block 990, additional LUT values L_(xi) and L_(yi) which arelocated intermediate the first 36 values determined are empirically byre-executing the second loop for 36 additional values. For example, ifthe values determined in the first execution of the second loop were forthe angles φ₁ =0,10,20,30, . . . , 340 and 350 degrees, then theintermediate angles determined in the second execution of the secondloop would be φ₂ =5,15,25,35, . . . , 345 and 355 degrees.

A third loop comprising steps 978, 982, 986 and 990 is re-executed untilthe error value is substantially zero or until all of the LUT locationshave been empirically determined. Control is then passed via path 994 tothe end of the calibration procedure.

In a preferred embodiment, the total number of positions represented bythe LUTs is approximately 16,000. The starting and stopping of therotation of the electron beam indicated in blocks 938 and 942 betweensuccessive calibration locations serves at least two functions. First,excessive heating of the target anode on the X-ray tube is prevented bybecause the rotating electron beam does not strike any one spot on theanode for an extended period of time. Second, hysteresis effects in thesteering coils are automatically compensated by continuous passagethrough complete hysteresis cycles. It will be understood that the abovecalibration procedure can either be performed manually under operatorcontrol or automatically under computer control.

Due to the finite amount of time required for the signals from theposition encoder on the rotating detector to arrive at the LUT and thecorresponding LUT values to drive the deflection coils on the X-raytube, there may be a time differential or lag between the time theposition of the rotating detector is sensed and transmitted to the LUT'sand the time the corresponding deflection data is transmitted from theLUT's to the X-ray tube deflection coils. At very slow or zero rotation,this lag is insignificant. However, as the rotation rate increases, thelag becomes greater and greater. This lag may be compensated for by aphase offset inserted between the position encoder and the LUT. Theoptimum phase offset is determined by varying the offset whileevaluating the focus of the image 830. For other than optimum offsets,the image will be blurred. The optimum offset will correspond to thesharpest image while the detector is rotating at a constant speed.

It will be understood that other calibration procedures may be used tosynchronize the rotation of the X-ray source and detector.

COMPUTER CONTROL AND ANALYSIS SYSTEM

FIG. 11 is a block diagram of the computer control and analysis systemarchitecture for the automated laminography inspection system of thepresent invention. The computer system is centered about the mastercontrol computer 270. A video frame grabber 1002 is incorporated intothe computer 270 via a plug-in board. The low light level camera 258 isconnected to master computer 270 via the line 276. A variety ofsubsystems, including X-ray 1004, motion control 1006, operator 1008,safety 1010, and printout 1012 communicate with the master computer viacommunication lines 1005, 1007, 1009, 1011 and 1013, respectively.Multiple high speed image analysis computers 272a, 272b, 272n, alsocalled "analysis engines", communicate with the master computer via thedata network 274. These communications take the form of "messages" thatare passed between the master computer and the analysis engines via thedata network 274. The analysis computers 272 also communicate with theframe grabber 1002 via a communication line 1014. In a preferredembodiment, each analysis computer 272 comprises a COMPAQ® 386 processorboard with an 80386 CPU, 5 megabytes of main RAM memory and a videoframe grabber memory. The master computer 270 also comprises a COMPAQ®386 processor board with an 80386 CPU. The analysis computers 272 areconnected to the master computer 270 by a standard SCSI network.

In operation, the master computer 270 controls the operation of theinspection system through the various subsystems 1004 through 1012. Themaster computer also controls the acquisition and analysis of thelaminographic images from which is derived a measure of the quality ofthe item under inspection. The master computer automatically controlsthe operation of the invention in two ways. First, a programmed sequenceof movements is executed to acquire digital cross-sectional imagesSecond, a programmed analysis procedure automatically examines andinterprets the digital cross-sectional images. The analysis of one imagemay be performed simultaneously with the acquisition of a second image.The analysis performed by the master computer system results in anoutput data listing which categorizes the various defects and otherconditions that were detected in the item under examination.

Specifically, for the inspection of solder joints on printed circuitboards, as illustrated in FIGS. 3a and 3b, the computer controls themotion of the XYZ positioning table 230 to which the circuit board 210is mounted. Often the area contained within one cross-sectional image,for example 0.400 inch×0.375 inch, is smaller than the total area of thecircuit board or other item to be inspected. In this case, the item islogically represented by multiple XY fields of views which, whencombined, include the total inspectable area of the circuit board. Themaster computer positions each XY field of view for inspection byissuing appropriate motion commands to the XYZ positioning table. Afterthe first XY field of view is in position for inspection, the resultingcross-sectional image is acquired and integrated in the camera. Thevideo signal of the image is then transmitted from the camera to thehigh speed image analysis computer 272. The circuit board may also bemoved to specific Z locations in order to bring different planes of thesolder joints into focus in the resulting cross-sectional images.

The preferred scan sequence for a circuit board is to collect all of therequired Z level images for a fixed XY location, then move to the nextXY location and collect all of the required Z level images for thatlocation. This step and repeat sequence iterates until all necessaryareas and levels of the board have been imaged and analyzed.

The fully automated inspection of all solder connections on a circuitboard, performed under the control of the master computer, utilizes apreprogrammed inspection routine, custom tailored for the specificcircuit board design being inspected. The board is scanned, and eachsolder connection is examined through the acquisition and analysis ofcross-sectional images.

A flow chart illustrating the steps of this automated inspection routineis shown in FIG. 12. Beginning in activity block 1050, a circuit boardfor inspection is inserted into the load/unload port 292 of theinvention (see FIG. 3c). Control is then transferred via path 1052 toactivity block 1054 wherein the master computer sends a message to theXYZ positioning table which causes it to move the circuit board into thefirst XY view location.

Proceeding via path 1056, the routine enters a first loop comprisingactivity blocks 1058, 1062, 1066, 1070 and 1074. In activity block 1058,the master computer receives a message that the board is at the firstview location. The master computer then controls the X-ray and detectorsubsystems such that a cross-sectional image of the board at thatlocation is acquired. After the cross-sectional image is acquired,control passes via path 1060 to activity block 1062 wherein thepreviously acquired cross-sectional image is sent to one of the analysiscomputers.

Proceeding via path 1064 to activity block 1066, a message is receivedby the analysis computer which uniquely identifies the view and slicerepresented by the received image. The image is then analyzed by theanalysis computer, while the master computer program proceeds via path1068 to decision block 1070. In block 1070, the identity of the mostrecently acquired slice is checked to see if that is the last Z slice tobe taken at that XY view location. If more Z slices are required,control passes via path 1072 to activity block 1074. In block 1074, theXYZ positioning table moves the circuit board in the Z direction thuspositioning it for the next Z slice to be acquired. Control thenproceeds via path 1076 back to activity block 1058. Anothercross-sectional image is acquired in block 1058, which is sent to ananalysis computer in block 1062, and identified and analyzed in block1066. The first loop, comprising the steps 1058, 1062, 1066, 1070 and1074, is repeated until it is determined in decision block 1070 that thelast Z slice for the current XY view position has been acquired.

When the last Z slice has been acquired, control is transferred via path1078 to activity block 1080 wherein a message indicates that theinspection of that particular XY view is complete. For example, if aparticular XY view requires three different Z level slices, then thefirst loop will be executed three times, once for each Z level. At thecompletion of the third execution of the first loop, a message indicatesthat all data for that XY view has been acquired and analyzed.

A timing diagram for the steps identified as the first loop is shown inFIG. 13. The unit of time chosen is one frame time, or 1/30 second,which is the rate at which the images are transmitted by the camera asvideo signals. At the start of the first loop cycle, the circuit boardis positioned at the desired inspection location, the X-rays are on, andthe camera begins to integrate the image for three frame times (0.1second). During this 0.1 second, the turntable 256 and X-ray source 280(FIG. 3a) make one complete revolution. During the next consecutiveframe time, beginning at time 3/30 seconds, the image is "grabbed" fromthe camera 258 and sent to one of the image analysis computers 272 (FIG.11). Meanwhile, the master computer 272 (FIG. 11) executes a firstcommand which stops the production of X-rays (This is accomplished bydirecting the electron beam 285 into the beam stop 360 in FIG. 4.) and asecond command which moves the circuit board to the next view area orslice position for acquisition of another image. This movement istypically completed within 0.1 second. During this 0.1 second, thecircuit board is moved to the next position and stopped. The system ispreferably designed so that any mechanical vibrations caused by themovement will be substantially dampened before the end of the 0.1 secondtime period. The computer then executes a command which causes X-rayproduction to resume and the cycle is repeated. Typical cycle time forthe acquisition of a single image is therefore approximately 0.2 second,corresponding to a speed of five images per second.

Even though the time required by the computer to completely analyze animage may exceed the 0.2 second image acquisition cycle time, oneembodiment of the invention still performs real time image processing byutilizing the parallel processing analysis computers 272 shown in FIG.11. The parallel processing architecture enables the system to performseveral different activities simultaneously. For example, the system maysimultaneously analyze several different images while also acquiringadditional images. Thus, the system does not need to wait for each imageanalysis to be completed before subsequent images can be acquired. Theoptimum number of analysis computers can be determined, based upon thecomplexity of the image analyses being performed, such that the imageprocessing computing does not become a bottleneck in the inspectionprocess.

Upon completion of an XY view in block 1080, control is transferred viapath 1082 to activity block 1084, wherein the results for thatparticular XY view inspection are stored in the memory of the mastercomputer Proceeding via path 1086 to decision block 1088, the XY viewidentification is checked to determine if additional XY views of thecircuit board are required.

If additional XY views are required, then control is transferred viapath 1090 to activity block 1054. A second loop comprising steps 1054,1058, 1062, 1066, 1070, 1074, 1080, 1084 and 1088 is executed multipletimes until all of the programmed image locations on the circuit boardhave been acquired and analyzed.

When all of the programmed image locations have been inspected, controlis transferred via path 1092 to activity block 1094 which indicates thatthe inspection is complete and it is time to unload the board.

Proceeding via path 1096 to activity block 1098, the inspection resultsfor the previously inspected board are output in the form of aninspection report. Control then passes via path 1100 back to thebeginning of the inspection routine at activity block 1050 and thesystem is ready to begin inspection of another circuit board.

An example of a typical inspection report is shown in FIG. 14. Variousbookkeeping entries record the date and time 1102 of the inspection, themodel number of the circuit board 1104 and the serial number of thespecific board inspected 1106. Results of the inspection are tabulatedin three columns which identify the device name 1108, the pin numberwhere defects were identified 1110, and the type of solder defectidentified. In this particular example, it is seen that on a deviceidentified as U13, there is a solder bridging defect between pins 2 and3. Similarly, device R17 has insufficient solder at pin 1. The devicesU13, R2, R17, etc. are typically electronic devices such as integratedcircuit chips, resistors, capacitors, etc. Additionally, the inspectionreport may provide statistical summaries providing trend analysis ofvarious defects and process control parameters. The inspection reportmay also include operation summaries showing the chronological historyof the machine operation during some past period of time. The operationsummaries may include a report of machine utilization factors includingthe identity of the operators; start times, stop times and dates foreach operator's duty shift; and the number of boards processed duringeach shift.

The total time required to inspect an entire circuit board, utilizingthe above described routine, is determined by several factors. Three ofthese factors are (1) the number of slices (cross-sectional images) atdifferent Z levels needed for each XY view location, (2) the field ofview size, i.e., the area covered by each individual image and (3) thesize of the circuit board, i.e., the total area to be inspected.

A typical circuit board inspection may require anywhere from one toeight Z slices at each XY location, depending upon the complexity of thedevices on the board and the type of solder connections. The field ofview is the inspectable area acquired per image, and in one embodimentof the invention is approximately 0.400 inch by 0.375 inch. This fieldof view size results in high resolution images wherein each pixel hasdimensions on the order of 0.0008 inch. Finally, the number of XY viewsand Z slices required to scan a particular circuit board, mosaicfashion, will determine the total number of views required and hence,the total amount of time required for the inspection.

For example, a 6"×9" circuit board (54 square inches), might have 50square inches of area requiring inspection. At 0.15 square inches perfield of view (0.400 inch times 0.375 inch), approximately 360 XY fieldof view locations are required to cover the entire board. Assuming, thatan average of two Z slices are required at each location, thisparticular circuit board will require 720 images for a completeinspection. At the rate of five board would be approximately 144seconds.

Typical inspection times may range from 20 seconds for very simplecircuit boards up to 8 minutes for larger, more complex boards requiringhigh resolution inspection.

AUTOMATED SOLDER CONNECTION DEFECT ANALYSIS

The present invention is particularly will suited for performingautomated inspections of the solder connections between electroniccomponents mounted on circuit boards. In one embodiment, this isaccomplished by acquiring high resolution X-ray cross-sectional imagesof the solder connections and analyzing the images by means of acomputer controlled digital image processing procedure. There arepresently a multitude of different types of solder connection defectswhich may be analyzed in this manner. However, the general concept ofautomated solder connection image analysis may be illustrated by a fewillustrative examples. Such examples include bridging of solder betweenadjacent connection points, insufficient quantity of solder at aconnection and missing solder at a connection.

FIG. 15 shows a portion of a typical circuit board 210 upon which arelocated multiple electronic devices 212 and 1150 interconnected bymultiple solder connections 214. In order to simplify the explanation ofthe automated analysis procedures, a specific type of electronic deviceand corresponding solder connection will be singled out for detaileddiscussion. However, it will be understood that the invention is not tobe limited by the specific device chosen and that the invention appliesto numerous other types of devices, technologies and electricalconnections. Specifically, a device employing surface mount technologywill be described in detail, however, the invention is also applicableto many other types of circuit board technologies includingplated-through-hole technology.

Surface Mount Technology (SMT) is a widely used technique whereinelectronic devices comprising metallized connector pads are soldered tocorresponding metallized connector pads on the surface of a circuitboard. FIG. 16 illustrates a typical SMT device 212 shown in an elevatedposition over the mounting location on the circuit board 210 to which itwill be connected. Specifically, the electronic device 212 comprises apackage commonly used in the electronics industry and as known in thetrade as a Leadless Chip Carrier (LCC). The LCC 212 comprises multiplemetallized connector pads 1160a, 1160b, 1160c, . . . , 1160n which, whenthe LCC is located in position on the circuit board 210, are locatedimmediately adjacent corresponding metallized circuit board connectorpads 1260a, 1260b, 1260c, . . . , 1260n, respectively. The metallizedpads 1260 are formed on or near the surface of the circuit board 210 andprovide the electrical connection points for interconnecting the variouselectronic devices 212 and 1150 comprising the completed circuit boardassembly.

FIG. 17 is an enlarged view of a portion of the LCC 212 illustrating thegeneral visual appearance of solder connections formed between the fivemetallized connector pad pairs 1160a/1260a through 1160e/1260e. Solderconnection 1360e formed between pads 1160e and 1260e is an example of agood connection having no visible defects. A solder bridging defect 1370is shown between adjacent solder connections 1360a and 1360b. Aconnection 1360c having insufficient solder is shown between pads 1160cand 1260c. A solder connection 1360d visually appears to have no defectsbut comprises internal voids. There is no solder shown at a connection1360f between pads 1160f and 1260f.

FIG. 18 illustrates the appearance of an X-ray cross-sectional image ofthe portion of the LCC device 212 shown in FIG. 17. The plane shown bythe cross-sectional image is parallel to the plane defined by thecircuit board 210 and approximately 0.0005 inch above the surface of thecircuit board. The phantom lines indicating the location of the device212, device connection pads 1160 and circuit board connection pads 1260are shown for reference purposes only and may not be present in anactual cross-sectional image Image regions 1360a', 1360b', 1360c',1360d', 1360e' and 1370' correspond to the solder connections 1360a,1360b, 1360c, 1360d, 1360e and defect 1370, respectively, in thedesignated image plane.

IMAGE ANALYSIS FOR DETECTION OF SOLDER BRIDGING DEFECTS

A solder bridging defect is the presence of unwanted solder betweentraces on a circuit board, between a connection pad and a trace, betweentwo separate connection pads, or between two separate connection pins.An enlarged portion of FIG. 18 at the location of the bridging defectimage 1370' between connection pads 1260a and 1260b is shown in FIG. 19.An arbitrary pixel grid comprising columns and rows is also shown to aidin the description of the automated procedure for detecting bridgingdefects.

Each pixel in the image is associated with an intensity value whichcorresponds to the optical density of the image represented by thatpixel. The intensity values form a gray scale which ranges from zero(black) to 255 (white). The images of high density materials whichreadily attenuate X-rays, e.g., solder, are represented by relativelylow intensity values corresponding to the darker shades of gray near theblack end of the gray scale. Conversely, low density materials, e.g.,plastic circuit boards, produce images having intensity valuescorresponding to the lighter shades of gray near the white end of thegray scale. Images having this type of gray scale are known as"positive" images. It will be understood that the relationship betweenshades of gray and intensity may be reversed to produce what arecommonly known as "negative" images. Either negative or positive imagesmay be used in the invention, but for purposes of explanation, positiveimages will be used. Therefore, pixels within the regions of the imagerepresenting solder material, e.g., regions 1360' and 1370', correspondto relatively low image intensity values. Pixels in other regions of theimage represent lower density materials, e.g., plastic circuit board,and correspond to relatively high image intensity values.

The initial step in the image analysis comprises acquiring topographicaldata and inspection parameters necessary for performing an inspectionand evaluation of a solder bridging defect. In one embodiment of theinvention, a data file contains this specific information for each imageanalysis being performed. Once the circuit board is identified, the datafile for that specific type of board is recalled and placed in theanalysis computer(s) memory. An algorithm for analyzing an image for thepresence of a bridging solder defect uses as input, the centroidlocation and boundaries of the circuit board connection pad 1260, apredetermined search path location and a predetermined differential grayvalue threshold. For the example shown in FIG. 19, the data file willcontain the information that the centroid 1378 of connection pad 1260ais located at column and row pixel coordinates (C50,R75). Additionally,the data file will contain the information that the pixel width of pad1260a is the difference between pixel column numbers C75 and C25 andthat the pad length is the difference between pixel row numbers R125 andR25. Any other inspection parameters, such as the differential grayvalue threshold and search path location and dimensions, necessary forperforming the bridging solder defect analysis will also be retrievedfrom the data file.

The procedure for analyzing the cross-sectional X-ray image of a solderconnection for a solder bridging defect is illustrated in FIG. 19 withrespect to solder connection 1360a, Preferably, the plane of thecross-sectional image lies in a plane which is substantially parallel tothe circuit board plane and is approximately 0.0005 inch above thesurface of the circuit board. The procedure generally comprisesdetermining, from the image, the presence of unwanted solder along asearch path which completely surrounds the solder connection ofinterest.

Using the topographical data for pad 1260a, an analysis algorithmproceeds to define a search path 1380 around the boundaries of the padcomprising path segments 1380a, 1380b, 1380c and 1380d. The search pathis one pixel in width and is positioned at a predetermined distance fromthe boundaries of the pad. In the embodiment wherein a digital imagecomprises 512 columns and 480 rows and corresponds to an area on thecircuit board of approximately 0.400 inch×0.375 inch, one pixel widthcorresponds to a distance of approximately 0.00078 inch on the circuitboard. The predetermined distance between the pad boundaries and thesearch path in FIG. 19 is the difference between pixel columns C95 andC75 and the difference between pixel rows R5 and R25. The predetermineddistance may be selected empirically to meet the requirements of anyparticular analysis application.

The image intensity of each pixel comprising the search path 1380 iscompared to the intensity of the adjacent pixels in the search path todetermine a differential gray value ΔG. The image intensity or grayvalue of a particular pixel is given by I_(C),R. The differential grayvalue ΔG₁,2 between two adjacent pixels 1 and 2 is then found by takingthe difference between their respective intensities I₁ and I₂. Eachdifferential gray value ΔG₁,2 is then compared to a predeterminedthreshold value ΔG_(Th). The threshold value is selected to indicatewhen one pixel is located in a solder portion of the image and theadjacent pixel is located in a circuit board portion of the image. Thepresence of unwanted solder along the search path is indicated when adifferential gray value exceeds the threshold value.

By way of example, consider a search beginning at a corner 1382 of thesearch path 1380 located at a first pixel (C95,R5) having an intensityI₁ and proceeding up column C95 to the next adjacent pixel on pathsegment 1380a to a second pixel (C95,R6) having an intensity I₂. It isto be understood that this starting position is arbitrary and that anyother position along the search path could also be chosen to begin thesearch. The differential gray value for these first two adjacent pixelsis given by

    ΔG.sub.1,2 =I.sub.1 -I.sub.2 =I.sub.C95,R5 -I.sub.C95,R6(7)

If the absolute value of the differential gray value |ΔG₁,2 | is greaterthan or equal to the threshold value ΔG_(Th), then the location of thepixels and the sign, i.e., positive or negative, of the differentialgray value are stored as candidate defect indications D_(i), where i isan integer corresponding to the order in which the defect indication wasfound. For example, D₁ corresponds to the first defect indicationencountered along the search path 1380 from the starting position 1382,D₂ corresponds to the second defect indication encountered and so on.

In the example shown in FIG. 19, a first defect indication D₁ is foundat a pixel K, located approximately at (C95,R55). If pixel K is in thesolder defect 1370' portion of the image, then the previous pixel K-1 inthe search path, located approximately at (C95,R54), is approximatelyoutside the solder portion and will have a higher intensity value thanthe pixel K. Therefore, an appropriately selected ΔG_(Th) will besmaller than the absolute value of a differential gray value |ΔG_(K-1),K| derived from the intensities I_(K-1) and I_(K) of these two adjacentpixels K-1 and K. Additionally, ΔG_(K-1),K is positive in sign.Similarly, a second defect indication D₂ is found at a pixel M locatedapproximately at (C95,R90). If pixel M is in the solder defect 1370'portion of the image, then the subsequent pixel M+1 in the search path,located approximately at (C95,R91), is outside the solder portion andwill have a higher intensity value than the pixel M. Therefore, theabsolute value of a differential gray value |ΔG_(M),M+1 | derived fromthe intensities I_(M) and I_(M+1) of these two adjacent pixels M and M+1is greater than ΔG_(Th). Additionally, ΔG_(M),M+1 is negative in sign.The presence of the bridge defect 1370' is thus revealed when defectindication D₁ is positive and the next defect indication D₂ is negative.

The search for defect indications continues around the path 1380 untilthe entire path has been examined. A report of all the bridges found isthen recorded and reported.

A flowchart illustrating the process for automatically locating solderbridging defects is shown in FIG. 20. Beginning in an activity block1400, the topographical data and other inspection parameters for theparticular connection pad being analyzed are recalled from the analysiscomputer's memory. Proceeding via a path 1402 into an activity block1404, the search path around the connection pad is defined utilizing thetopographical data and other inspection parameters stored in the memoryof the computer. Control is then transferred via a path 1406 to activityblock 1408 wherein the search path scan is initialized by setting apixel counter "i" and a defect indication counter "j" equal to one.

A first loop comprising activity blocks 1412, 1416, 1420, 1424, 1428 and1434 is entered via a path 1410 from activity block 1408. In the firstloop, every pixel comprising the search path is examined, differentialgray values are calculated and candidate defect locations are identifiedand stored for further processing at a later time. In the first activityblock 1412 of the loop, the differential gray value ΔG₁,2 for the firstand second pixels in the search path is calculated. This value is passedvia a path 1414 to a decision block 1416 wherein the absolute value ofthe differential gray value |ΔG₁,2 | is |ΔG₁,2 | is greater than orequal to ΔG_(Th), control passes via a path 1418 to activity block 1420.In activity block 1420, the locations of pixels 1 and 2 and the sign ofΔG₁,2 are stored as a first defect indication D₁. Control passes toactivity block 1424 via a path 1422 wherein the defect counter "j" isincremented by one. In a decision block 1428, the final block of thefirst loop, reached via a path 1426, a completion check is performed todetermine if the entire search path has been examined. If not, controlpasses via a path 1432 to activity block 1434, wherein the search pathpixel counter "i" is incremented by one. Control then returns via path1436 to the beginning of the first loop at activity block 1412. Thefirst loop is repeated until all of the pixels comprising the searchpath have been analyzed, at which time control passes out of the firstloop from decision block 1428 via path 1438 to activity block 1440.

In activity block 1440, the defect counter "j" is again initialized tothe value one prior to entering a second loop via path 1442. The secondloop comprises blocks 1444, 1448, 1452 and 1458. In the second loop, thedefect indications D_(j), identified in the first loop, are examined todetermine the locations of solder bridging defects along the searchpath. Entering the second loop at decision block 1444 with j=1, thesigns of defect indications D₁ and D₂ are determined. If D₁ is positiveand D₂ is negative, then control passes via path 1446 to activity block1448 wherein the locations of D₁ and D₂ are recorded and a solderbridging defect is recorded at the search path segment between D₁ andD₂. Control then passes via path 1450 to a decision block 1452 where acompletion test is performed to determine if all of the defectindications D_(j) have been analyzed. If not, control passes via path1456 to an activity block 1458 where the defect counter "j" isincremented by one. Control then returns via path 1460 to the beginningof the second loop at decision block 1444. The second loop is repeateduntil all of the defect indications D_(j) located along the search pathin the first loop have been analyzed for the solder bridging defect.Control then passes out of the second loop from decision block 1452 viapath 1462 to activity block 1464. In block 1464, a report of all thesolder bridging defects found along the search path is generated andstored for later recall.

IMAGE ANALYSIS FOR DETECTION OF MISSING OR INSUFFICIENT SOLDER DEFECTS

A missing solder defect is defined as the presence of substantially zeroor very small quantities of solder at a connection comprising anelectronic device connection pad and a corresponding circuit boardconnection pad. An insufficient solder defect is defined as some solderpresent at the connection, but not enough to form a proper fillet or toprovide sufficient strength to the connection. An enlarged portion ofFIG. 18 at the location of the insufficient solder defect image 1360c'between connection pads 1160c and 1260c is shown in FIG. 21. Anarbitrary pixel grid comprising columns and rows is also shown to aid inthe description of the automated procedure for detecting missing orinsufficient solder defects.

The procedure for analyzing the cross-sectional X-ray image of a solderconnection for a missing solder or insufficient solder defect isillustrated in FIG. 21 with respect to solder connection image 1360c'.Preferably, the plane of the cross-sectional image lies in a plane whichis substantially parallel to the circuit board plane and isapproximately 0.0005 inch above the surface of the circuit board. Theprocedure generally comprises determining, from the image, the thicknessof the solder connection in several specific regions.

Three specific regions of a solder connection are defined in referenceto FIG. 22. FIG. 22 is a cross-sectional view of a typical good solderconnection, such as connection 1360e. The cross-sectional view is alongthe line 22--22 in FIG. 17. A first region 1501 of the connection 1360e,sandwiched between the device connection pad 1160 and circuit boardconnection pad 1260, is designated the "pad" of the connection. A secondregion 1502, beginning approximately at a side wall 1505 of the device212 and extending approximately to a point 1506 between the wall 1505and a border 1507 of the pad 1260, is designated the "heel" portion ofthe connection 1360e. A third region 1503, beginning approximately atthe point 1506 and extending approximately to the border 1507 of the pad1260, is designated the "toe" portion of the connection 1360e.

Typically, the pad region 1501 comprises a nearly uniform thickness ofsolder which is relatively thin. The heel region 1502 is generally ofnon-uniform thickness and comprises the thickest portion of theconnection. The toe region 1503 is generally more uniform in thicknessthan the heel, but is not as thick. The amount of solder comprising theconnection 1360e can be estimated from measurements of the averagethickness of the solder in each of the three regions 1501, 1502 and1503.

In a laminographic cross-sectional image of solder material, typically acombination of lead and tin, there is a relationship between theintensity of the image and the thickness of the solder material formingthe image. FIG. 23a illustrates an example of this general relationship.In this example, it is seen that the image intensity decreases fromvalues corresponding to lighter shades of gray (white) to valuescorresponding to darker shades of gray (black) as the thickness of thesolder material increases. That is, the image of a thin section ofsolder will have an image intensity value that is greater than the imageintensity value of the image of a thicker section of solder. The imageof the thin section will appear to be a lighter shade of gray than theimage of the thicker section. This relationship may be calibrated byusing a calibration step wedge comprising multiple steps of differingthickness. An example of such a step wedge 1560 is shown in FIG. 23b.Step wedge 1560 is constructed of solder material and comprises tensteps 1571 through 1580 having thicknesses ranging from 0.001 inch to0.010 inch in increments of 0.001 inch. An X-ray laminographiccross-sectional image of the step wedge 1560 taken at a plane includingthe line 1590 and parallel to a base 1592 of the wedge exhibits theimage intensity versus solder thickness relationship shown in FIG. 23c.Since the thicknesses of the steps 1571 through 1580 are known, thecorresponding intensities 1571, through 1580, may be compared tointensities of other cross-sectional images of solder material where thethicknesses are not known to determine the unknown thicknesses.

The initial step in the analysis comprises acquiring topographical dataand inspection parameters necessary for performing an inspection andevaluation of a missing or insufficient solder defect. One embodiment ofthe invention provides a data file containing this specific informationfor each analysis to be performed. An algorithm for analyzing an imagefor the presence of a missing or insufficient solder defect uses asinput, the centroid location and boundaries of the connection pad, threeinspection windows and six threshold values. In this example, the datafile will contain the information that the centroid 1679 of connectionpad 1260c is located at column and row pixel coordinates (C100,R62) inFIG. 21. Additionally, the data file will contain the information thatthe pixel length of pad 1260c is t he difference between pixel columnnumbers C50 and C150 and that the pad width is the difference betweenpixel row numbers R75 and R50. Any other inspection parameters necessaryfor performing the analysis will also be retrieved from the data file.

Using the topographical data and inspection parameters for a missing orinsufficient solder defect analysis of the image 1360c, at pad 1260c,the image analysis algorithm proceeds to define the boundaries of threeinspection windows 1601, 1602 and 1603, shown in FIG. 21. Each window isrectangular in shape and is positioned at a predetermined distance fromthe boundaries and centroid of the pad. The first window 1601 is definedby four corners having pixel coordinates (C55,R55), (C55,R70), (C85,R70)and (C85,R55). Window 1601 substantially overlaps the pad region 1501 ofthe solder connection. The second window 1602 is defined by four cornershaving pixel coordinates (C95,R55), (C95,R70), (C120,R70) and(C120,R55). Window 1602 substantially overlaps the heel region 1502 ofthe solder connection. The third window 1603 is defined by four cornershaving pixel coordinates (C125,R55), (C125,R70), (C145,R70) and(C145,R55). Window 1603 substantially overlaps the toe region 1503 ofthe solder connection.

The average image intensity within a window is determined by summing theimage intensities of all of the pixels comprising the window anddividing by the total number of pixels contributing to the sum. Theaverage intensities thus derived from the pad region window 1601, theheel region window 1602 and the toe region window 1603 are designatedI_(P), I_(H) and I_(T), respectively. These average intensities, aspreviously discussed, are directly related to average thickness T_(P),T_(H), and T_(T) of the solder in each of the respective regions. Thepresence of a missing or insufficient solder defect is determined bycomparing these average thicknesses TP, TH, and T_(T) to predeterminedthickness threshold values Th_(M),P, Th_(M),H, Th_(M),T, Th_(I),P,Th_(I),H, and Th_(I),T. Generally, the missing solder threshold values,Th_(M),P, Th_(M),H and Th_(M),T, corresponding to the pad, heel and toeregions respectively, are smaller than the insufficient solder thresholdvalues, Th_(I),P, Th_(I),H, and Th_(I),T. That is, Th_(M),P <Th_(I),P,Th_(M),H <Th_(I),H and Th_(M),T <Th_(I),T. Specifically, if T_(P)<Th_(M),P, T_(H) <Th.sub. M,H and T_(T) <Th_(M),T, then the connectionis reported as having missing solder. If Th_(M),P <T_(P) <Th_(I),P,Th_(M),H <T_(H) <Th_(I),H and Th_(M),T <T_(T) <Th_(I),T, then theconnection is reported as having insufficient solder.

A flowchart illustrating the process for automatically locating missingor insufficient solder defects is shown in FIG. 24. Beginning in anactivity block 1700, the topographical data and other inspectionparameters for the particular connection pad being analyzed are recalledfrom the analysis computer's memory. Proceeding via a path 1702 into anactivity block 1704, inspection windows for the pad, heel and toeregions of the solder connection are defined utilizing the topographicaldata and other inspection parameters stored in the memory of thecomputer. Control is then transferred via a path 1706 to activity block1708 wherein the average image intensity within each window isdetermined and the corresponding average solder thickness is calculated. Control is then transferred to decision block 1712 via path 1710.

In decision block 1712, the average solder thicknesses T_(P), T_(H), andT_(T) within the windows are compared to the insufficient solderthickness threshold values Th_(I),P, Th_(I),H, and Th_(I),Trespectively. If the average thicknesses are not less than theinsufficient solder thresholds, control passes via path 1714 to the endof the analysis routine. If the average thicknesses are less than theinsufficient solder thresholds, control passes via path 1718 to decisionblock 1720. In decision block 1720, the average thicknesses T_(P),T_(H), and T_(T) are compared to the missing solder thickness thresholdvalues Th_(M),P, Th_(M),H, Th_(M),T respectively. If the averagethicknesses are not less than the missing solder thresholds, controlpasses via path 1722 to activity block 1724 wherein the presence of aninsufficient solder defect is recorded. Control then passes via path1726 to the end of the analysis routine. If the average thicknesses areless than the missing solder thresholds in decision block 1720, controlpasses via path 1728 to activity block 1730 wherein the presence of amissing solder defect is recorded. Control then passes via path 1732 tothe end of the routine.

The system and processes described herein were developed primarily forthe inspection of solder connections on printed circuit boards. However,the invention may also be useful for the inspection of other objects andfeatures. While the above description comprises one preferred embodimentof the invention as applied to the inspection of solder connectionsbetween electronic devices on printed circuit boards, there are otherapplications which will be obvious to those skilled in the art.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

We claim:
 1. A solder connection inspection device comprising:an X-raysource comprising:an electron source for producing a beam of electrons;an anode having a surface upon which said electron beam impinges; and anelectrical steering device for deflecting said electron beam todifferent locations on said anode surface, said steering device capableof causing said electron beam to rotate about an axis and to trace apath upon said anode surface thus causing said X-ray source to move in afirst pattern, said first pattern defining a first plane; an X-raydetector for producing an X-ray cross-sectional laminographic image of acutting plane of said solder connection, said detector positioned toreceive X-rays produced by said X-ray source which have penetrated saidsolder connection, said X-ray detector comprising:a converter screen forproducing an optical image corresponding to an X-ray image formed bysaid penetrating X-rays; means for moving said converter screen suchthat it rotates about said axis and travels along a path defining asecond pattern, said second pattern defining a second plane which issubstantially parallel to said first plane; an optical derotation devicefor transmitting said optical image from said rotating converter screento a stationary optical image plane; and a camera located at saidstationary optical image plane for detecting said optical image, saidcamera having an electronic output which corresponds to said opticalimage; a control system for synchronizing the motions of said X-raysource and converter screen, such that said optical imagerepresentations of said X-ray cross-sectional laminographic images areformed at said stationary image plane, said control system comprising:asensor which monitors the position of said converter screen along saidsecond pattern and transmits coordinates corresponding to the screenposition; and a look up table which receives said coordinates from saidsensor and transmits corresponding signals to said steering device thuscausing said motion of said X-ray source to be synchronized with saidmotion of said converter screen; and a digital image processing systemfor analyzing said cross-sectional images and deriving from saidcross-sectional images information which is indicative of features ofsaid solder connection, said image processing system comprising:an imagedigitizer for receiving said electronic image signal from said cameraand forming a digital representation of said image which corresponds tosaid X-ray cross-sectional image of said solder connection; and aprogrammably-controlled computational section programmed to accesspredetermined regions of said digital image and to analyze said regionsin accordance with a predetermined set of instructions for specificfeatures indicative of particular types of solder defects.
 2. Anapparatus as defined in claim 1, wherein said X-ray source comprises asteerable electron beam X-ray tube.
 3. An apparatus as defined in claim1, wherein said electrical steering device comprises coils which producemagnetic fields which interact with said electron beam causing it to bedeflected.
 4. An apparatus as defined in claim 1, wherein said converterscreen comprises a cadmium tungstate scintillation material.
 5. Anapparatus as defined in claim 1, wherein said cross-sectional imagecorresponds to a plane within said solder connection which is parallelto said first and second planes and intersects said axis of rotation. 6.An apparatus as defined in claim 5, wherein said image plane of saidsolder connection is located between said X-ray source and said X-raydetector such that the distance from said image plane to said firstplane is less than the distance from said image plane to said secondplane.
 7. An apparatus as defined in claim 1, wherein said means formoving said converter screen further comprises a turntable which rotatesabout said axis and upon which said converter screen is mounted.
 8. Anapparatus as defined in claim 7, wherein said optical derotation devicecomprises first and second mirrors mounted in said turntable, saidmirrors oriented at an angle of approximately 45° with respect to saidaxis of rotation and with respect to said first and second planes.
 9. Anapparatus as defined in claim 8, wherein said first mirror receives saidoptical image from said converter screen and reflects it to said secondmirror, said second mirror further reflecting said image to saidstationary image plane.
 10. An apparatus as defined in claim 8, whereinsaid second mirror intersects said axis of rotation.
 11. An apparatus asdefined in claim 1, wherein said camera comprises a low light levelvideo camera.
 12. An apparatus as defined in claim 11, wherein said lowlight level camera comprises a silicon intensified target imageintensifier.
 13. An apparatus as defined in claim 1, wherein saiddigital image processing system comprises multiple parallel imageprocessors.
 14. A laminographic apparatus for inspecting an electricalconnection comprising:a moveable source of penetrating radiation; asteering device for controlling the motion of said source; an imagingsystem for producing a cross-sectional image of a cutting plane of saidelectrical connection wherein said cross-sectional image displaysspecific features of said electrical connection, said imaging systemcomprising:a radiation detector having a variable position image formingregion wherein the position of said image forming region is monitored bya position sensor which transmits coordinates corresponding to itsposition; and a look up table which receives said coordinates from saidsensor and transmits corresponding signals to said steering device thuscausing said motion of said radiation source to be synchronized withsaid motion of said image forming region; and an image analysis systemfor analyzing said image to detect said specific features, wherein saidimage analysis system searches said cross-sectional image forpredetermined characteristics and performs predetermined analyticaltests on said cross-sectional image.
 15. An apparatus as defined inclaim 14, wherein said imaging system comprises an X-ray source and anX-ray detector.
 16. An apparatus as defined in claim 15, wherein saidimaging system produces X-ray laminographs of said electricalconnections.
 17. An apparatus as defined in claim 16, wherein said X-raylaminographs are produced by the motion of said X-ray source and saidX-ray detector with respect to said electrical connections.
 18. Anapparatus as defined in claim 17, wherein said motion of said X-raysource is produced by electrical means and said motion of said X-raydetector is produced by electro-mechanical means, said X-ray source andsaid X-ray detector motions being synchronized and controlled by anelectrical feedback system.
 19. An apparatus as defined in claim 17,wherein said motions of said X-ray source and said X-ray detector aresubstantially circular and define a source plane and a detector plane.20. An apparatus as defined in claim 19 wherein said source plane andsaid detector plane are substantially parallel.
 21. An apparatus asdefined in claim 14, wherein one of said specific features comprises asolder bridging defect.
 22. An apparatus as defined in claim 21, whereinsaid image analysis system calculates a series of differential imageintensity gray values along a border surrounding said solder bond andcompares said differential gray values to a predetermined threshold grayvalue.
 23. An apparatus as defined in claim 22, wherein said solderbridging defect is identified by said image analysis system at locationsalong said border wherein said calculated differential image intensitygray values exceed said threshold value.
 24. An apparatus as defined inclaim 14, wherein one of said specific features comprises the quantityof solder present at said connections.
 25. An apparatus as defined inclaim 24, wherein said image analysis system defines three regions ofsaid cross-sectional image corresponding to three different portions ofsaid solder connection, calculates an average image intensity for eachof said three regions and compares said average image intensities to afirst and a second set of predetermined threshold values.
 26. Anapparatus as defined in claim 25, wherein said image analysis systemidentifies a missing solder defect at locations where said averageintensities are less than both of said first and second sets ofthreshold values.
 27. An apparatus as defined in claim 25, wherein saidimage analysis system identifies an insufficient solder defect atlocations where said average intensities are less than said first set ofthreshold values and greater than said second set of threshold values.28. A method of inspecting an electrical connection between electricalcomponents mounted on a printed circuit board comprising the stepsof:producing a cross-sectional image of a cutting plane of saidelectrical connection which lies in a plane that is adjacent to asurface of said printed circuit board, wherein said step of producingsaid cross-sectional image further comprises the steps of producingX-rays with an X-ray source and detecting X-rays with an X-ray detector;searching said cross-sectional image for predetermined features of saidelectrical connection; and performing predetermined analytical tests onsaid cross-sectional image.
 29. A method as defined in claim 28, whereinsaid step of producing said cross-sectional image further comprises thestep of producing an X-ray laminograph of said electrical connection.30. A method as defined in claim 29, wherein said step of producing saidX-ray laminograph further comprises the steps of moving said X-raysource and said X-ray detector with respect to said electricalconnection.
 31. A method as defined in claim 30, wherein said step ofproducing said X-ray laminograph further comprises the steps of:movingsaid X-ray source by electrical means; moving said X-ray detector byelectro-mechanical means; and synchronizing said X-ray source and saidX-ray detector motions with an electrical feedback system.
 32. A methodas defined in claim 28 wherein said step of performing predeterminedanalytical tests on said cross-sectional image further comprises thestep of searching for specific features in said cross-sectional image.33. A method as defined in claim 32, wherein said step of performingpredetermined analytical tests further comprises the step of performingsaid predetermined analytical tests on said cross-sectional image atpredetermined locations of said image to identify said specificfeatures.
 34. A method as defined in claim 28 wherein said step ofperforming predetermined analytical tests further comprise the stepsof:calculating a series of differential image intensity gray valuesalong a border surrounding one of said electrical connections; andcomparing said differential gray value to a predetermined threshold grayvalue.
 35. A method as defined in claim 34, further comprising the stepsof identifying locations along said border wherein said calculateddifferential image intensity gray values exceed said threshold value anddesignating said locations solder bridging defects.
 36. A method foridentifyig features of a solder joint using X-ray laminographscomprising the steps of:producing a cross-sectional image of a cuttingplane of said solder joint comprising the steps of:providing a source ofX-rays wherein X-rays are produced by an electron beam impinging upon atarget; directing said electron beam to different portions of saidtarget, thus moving a location wherein said X-rays are produced by saidX-ray source; detecting X-rays produced by said X-ray source with anX-ray detector having a variable position image forming region; movingthe location of said X-ray detector image forming region; andsynchronizing the motion of said location of the production of saidX-rays with the motion of said X-ray detector image forming region witha feedback system; searching said cross-sectional image forpredetermined characteristics; and performing predetermined analyticaltests on said cross-sectional image.
 37. A method as defined in claim36, further comprising the step of positioning a test object betweensaid X-ray source and said X-ray detector such that said test objectforms an image on said X-ray detector.
 38. A method as defined in claim37, further comprising the step of driving said feedback system inresponse to the motion of said detector such that said image of saidtest object is located at a predetermined position on said X-raydetector as said X-ray detector and said location of X-ray productionmove relative to said test object.
 39. A method of determining thequality of an electrical connection formed between a component mountedon a circuit board and a connection pad on said circuit board comprisingthe steps of:producing a high resolution cross-sectional image of acutting plane of said electrical connection wherein said electricalconnection has a first volume and said image has sufficient resolutionto permit detection of a physical feature of said connection having asecond volume which is substantially smaller than said first volume ofsaid connection; searching said cross-sectional image for saidpredetermined physical feature of said electrical connection;identifying said predetermined feature found in said searching step; andanalyzing said predetermined feature to determine the effect it has onthe quality of said electrical connection.
 40. An apparatus forinspecting an electrical connection formed between a component mountedon a circuit board and a connection pad on said circuit board todetermine the quality of the electrical connection, said apparatuscomprising:an imaging system for producing a high resolutioncross-sectional image of a cutting plane of an electrical connectionwherein said electrical connection has a first volume and said image hassufficient resolution to permit detection of a physical feature of saidconnection having a second volume which is substantially smaller thansaid first volume of said connection; an image analysis system foranalyzing said high resolution image to detect said physical feature,wherein said image analysis system searches said high resolutioncross-sectional image for a predetermined characteristic and performs apredetermined analytical test on said cross-sectional image to determinea measure of the quality of said electrical connection.
 41. Anelectrical connection inspection device comprising:an X-ray sourcecomprising:an electron source for producing a beam of electrons; ananode having a surface upon which said electron beam impinges; and anelectrical steering device for deflecting said electron beam todifferent locations on said anode surface; an X-ray detector forproducing X-ray cross-sectional laminographic images of a cutting planeof an electrical connection, wherein said detector is positioned toreceive X-rays produced by said X-ray source which have penetrated saidelectrical connection, said X-ray detector comprising:a converter screenhaving a variable position image forming region for producing an opticalimage corresponding to an X-ray image formed by said penetrating X-rays;an optical transformation device for transmitting said optical imagefrom said variable position image forming region to a stationary opticalimage plane; and a camera for detecting said stationary optical image,said camera having an electronic output which corresponds to saidoptical image; a control system for synchronizing the motions of saidX-ray source and variable position image forming region such that saidoptical image representations of said X-ray cross-sectionallaminographic images are formed at said stationary image plane, saidcontrol system comprising:a sensor which monitors the position of saidvariable position image forming region and transmits coordinatescorresponding to said image forming region position; and a look up tablewhich receives said coordinates from said sensor and transmitscorresponding signals to said steering device thus causing said motionof said X-ray source to be synchronized with said motion of saidvariable position image forming region; and a digital image processingsystem for analyzing said cross-sectional images of said electricalconnection, said image processing system comprising:an image digitizerfor receiving said electronic image signal from said camera and forminga digital representation of said image which corresponds to said X-raycross-sectional image of said electrical connection; and aprogrammably-controlled computational section programed to accesspredetermined regions of said digital image and to analyze said regionsin accordance with a predetermined set of instructions for specificfeatures of said electrical connection.
 42. A method for determining thequality of an electrical connection comprising the steps of:producing across-sectional image of a cutting plane of said electrical connection;searching said cross-sectional image for a specific feature inaccordance with a predetermined set of instructions; and analyzing saidspecific features in order to determine the quality of said electricalconnection.
 43. An apparatus for measuring a physical characteristic ofan electrical connection comprising:a radiation source for providing abeam of transmissive radiation; an imager which detects transmissiveradiation which is transmitted through said electrical connection andgenerates a cross-sectional electronic image of a cutting plane of saidelectrical connection; an image converter for converting said electronicimage into a gray scale coded image; and an image processor comprising alibrary of analysis algorithms and predetermined inspection parameterswherein said processor receives said gray scale coded image, analyzessaid gray scale coded image utilizing said analysis algorithms from saidlibrary thus performing predetermined computational analysis on saidgray scale coded image based upon each selected analysis algorithm, andprovides an output corresponding to a physical characteristic of saidelectrical connection.
 44. A method of detecting and characterizing astructural feature of an electrical connection, comprising the stepsof:exposing an electrical connection to a beam of transmissive radiationhaving sufficient energy to penetrate said electrical connection;detecting transmissive radiation which is transmitted through saidelectrical connection to create an electronic cross-sectional image of acutting plane of said electrical connection; converting said electroniccross-sectional image into a gray scale coded image; providing ananalysis algorithm; and performing a predetermined computationalanalysis on said gray scale coded image using said analysis algorithm.